Troubleshooting Common Furnace Problems With an HVAC Expert

Updated on May 16, 2019

Dan has been in the HVAC industry for 23 years with experience ranging from installation and service to sales and distribution.

Figuring Out Furnace Problems

Before you call out the pros, why not take a shot at fixing that furnace yourself?

I have been in the HVAC industry for over 23 years. There are a lot of things that can go wrong with a furnace, but many issues are easy for anyone to figure out and fix. Even if you need a part, don't be discouraged—many of the most commonly replaced parts are easy to install and can usually be bought from a local supplier. Your odds for getting the heat back on sooner rather than later are actually not too bad. Below, you'll find:

A furnace troubleshooting checklist.
Tips on common furnace repairs.
A video guide to help you fix it yourself.

Furnace Troubleshooting Checklist

Below, I explain each of these processes in more detail. We start with the simplest problems and then test for more difficult issues. It's important to be methodical and systematic in your diagnosis.

Check the thermostat settings.
Check for power via the breaker, switch, indicator light, and fuse.
Check that the inline gas valve is open.
Check filter.
Check ignitor for ignition.
Check flame sensor for ignition confirmation.
Check for flue/chimney blockage: pressure switch.
Know when to call a service technician.

1. Check the Thermostat

I know it may sound silly, but before we get carried away, let's check the thermostat for a couple of things. You might be surprised how much money per year is spent having guys like me come and turn up peoples' thermostats or replace their batteries.

Thermostat Problem Checklist

Check...	It Might...
for a jumbled digital display	need replacing (we can test this to be sure)
for a flashing battery indicator	need new 'AA' batteries
temperature setting	not be turned up high enough
function selection	not be turned to "heat" (switch off and on again to be sure)
inside for loose or touching wires	just need tightened or tidied up

Sometimes big problems have easy fixes.

How to Test a Thermostat

Testing your thermostat is really easy. All you need is a small or medium sized screwdriver and a short (6") piece of 18 gauge wire (what we'll call a "jumper wire"). If you go to the hardware store, ask for "18/2."

Simply go to the furnace, remove the doors, and follow these steps.

Locate the circuit board where the thermostat wires connect to the furnace.
Disconnect the wires connected to the "R" and "W" terminals. (The wires should be red and white, but aren't always.) Push them safely to the side.
Connect your jumper wire between the "R" and "W" terminals.
Re-secure the door and see if the furnace works.

Did it work?

NO: Then your thermostat isn't the problem.

YES: Then it's likely your thermostat is bad and needs replacing. It could be the wires connecting the thermostat to the furnace, but if they aren't loose, brittle, or broken, it's unlikely.

NOTE: Do not leave the jumper wire on the furnace to run it. It will make the furnace run continually without any temperature control, which can be extremely dangerous and cause other problems. Only use the jumper wire to see if the thermostat is faulty.

ALSO: These instructions are for testing a regular 24v (low voltage) circuit only. In other words, don't use these instructions if you're troubleshooting a line voltage circuit. (120v or higher)

Mercury-Controlled Thermostats Gotta Go

If you have an old thermostat that senses the temperature with a mercury bulb, get it replaced as soon as possible. It's not nearly as accurate as newer digital models and since it contains mercury, it's not safe.

Replacing Your Thermostat

If you decided you need to replace your own thermostat after performing the steps above, or you have an old mercury controlled version, you'll need to select a thermostat to use.

Most reputable thermostat makers will provide very clear directions with their units on how to install them, and it really isn't too difficult for the homeowner to do themselves if you take your time and follow the supplied directions.

You can get all fancy and buy a Nest thermostat that connects with your Alexa and all the other modern home-automation devices. Or, if you're like me, you just want something simple, easy to install, and easy to read. I prefer something like this Orbit Clear Comfort Thermostat (below).

Orbit 83521 Clear Comfort Programmable Thermostat with Large, Easy-to-Read Display

I install this Orbit Thermostat quite often. What I like is that it's still a more "basic" thermostat underneath the skin but it's a bit more modern looking from the oustide. So you get the modern look without all the complexity of a Nest or other new-age thermostat. It has well written instructions and good support for homeowners installing it themselves. The button below will take you to it's listing on Amazon.

Buy Now

2. Check for Power to the Furnace

If you hear the furnace trying to light (ignite) or hear the blower running but aren't getting any heat, then you can skip this step, because you have power. If not, then check the following:

Did a breaker trip?
Is the service switch at the furnace on? (Most units have this mounted to the furnace or ceiling just above.)
Is the little red light on the furnace's circuit board on? (Most modern furnaces have an indicator light to tell you it's getting power, and if you're lucky, tell you what might be wrong.)

Got power?

YES: Okay, then let's move on.

NO: Then you may need an electrician or HVAC professional. Before you call, though, let's check the fuse on the circuit board.

Did the Furnace Blow a Fuse?

Most modern furnaces have 3 amp fuses on the circuit board. However, the fuses might be located elsewhere on the boards in the case of some furnaces. It looks just like the standard size fuse you find in your car and should be easy to find.

If you have an indicator light and it's not on or flashing, this may be your problem and is easy to determine and fix.

Simply pull the fuse off the board and look to see if the link inside is burned and/or broken. If so, you should be able to pick one up at the local hardware or automotive store. If you don't already, it's just a good idea to keep extra fuses on hand since this issue is so common. It's usually not required, but if you can, get some 3 amp fast-acting fuses. These Bussmann brand fuses are perfect for furnaces.

DO NOT try to substitute the fuse with one of another amp rating.

3. Make Sure the Gas Is On

Sounds pretty simple, but it's certainly worth a check. I've been on calls where kids playing around the furnace have turned knobs and flipped switches, costing their parents a lot of grief (and about $150).

Did You Check the Filter? (I Had to Ask.)

Though it doesn't cause as many problems for heat as it does A/C, did you check the furnace filter?

The Furnace's Red Light Is Your Friend: A flashing red light helps you figure out the problem. | Source

4. Check the Flashing Red Light

If there's a little indicator light on your furnace's circuit board (usually visible through a tiny window on the furnace door) and it's flashing, it's trying to help.

This light blinks a sort of Morse code, a number that corresponds to a chart located somewhere on the furnace. It's usually on the doors that you remove from the front. For example, a "dot...dot...pause...dot...dot...pause" would be the number 2, which the chart may say indicates something like "ignition failure" or "pressure switch open."

Understanding the red blinking light on your furnace will help you navigate through the following possible issues that have a possibly simple solution.

5. Check for Blockages

Furnace Pressure Switch Problems

Sometimes, the little red light indicates an open pressure switch, which often means a flue/chimney blockage. It's also possible the pressure switch is bad, but before we decide that, let's see if the chimney is blocked. Birds and other small animals have been known to take up residence in flues when it's particularly cold. Check for this (and/or icing) by performing the following:

Remove the chimney pipe from the top of the furnace to see if there's anything in there. If you have a high-efficiency furnace, this may be difficult, but animals are less likely for you. Also, check outside to make sure the ends of the pipes are clear and not iced over.
Make sure the little fan hooked to the flue (the inducer fan) is working. If not, you'll probably need a service tech.

DO NOT get up on the roof to check the chimney unless you are completely confident and comfortable in doing so. It's not worth the risk since usually, the problem isn't up there. On occasion, birds will build a nest in the chimney, but if you haven't had problems so far this winter, it's not likely your problem. This is something that becomes apparent early in the heating season.

This is what a hot surface ignitor looks like when it's working. | Source

6. Check Furnace Ignition

This is easy:

Watch and listen to your furnace as it begins to cycle. Just before you hear the gas valve click open, you should see the hot surface ignitor begin to glow or hear the click, click, click of your spark ignitor. (You might even see this spark.) If you don't see or hear these things, the furnace will stop the cycle.
If your ignitor isn't working, the gas valve won't stay open until you replace the ignitor because the flame sensor is telling the valve that the gas isn't being lit.

NOTE: The igniter part can be tricky because it could be the pressure switch that won't let it light so you may have to run some checks on that switch to be sure which is causing the issue.

If the igniter glows or sparks, then make sure the gas is on. I have been on calls where kids playing around the furnace have turned knobs and flipped switches, costing their parents grief and about $150, so be sure the gas is turned on as well.
If the ignitor is working, but the gas valve didn't open, I am inclined to have you call a heating and cooling technician to work with a gas valve. This is not a simple repair and can cause dangerous issues if not done properly.
If you have a pilot light, look to see that its flame is lit. If not, this is likely your problem. Try to relight it. For an easy guide on this (with pictures), see my article about how to light an old water heater pilot. If it doesn't light, then you probably need a new thermocouple. These are cheap and can be bought at most hardware stores. Just make sure to measure how long of one you need before you go.

A Furnace Flame That Keeps Going Out

If your furnace lights the burners and then shuts off just a few seconds later, this is almost surely a fault of the flame sensor. This is one of the most common furnace problems homeowners face.

Cleaning and/or replacing a flame sensor is one of the most common and simple heating and air conditioning repairs one can make. Without a doubt, you should give this a try on your own before calling an HVAC technician.

Calling a HVAC Professional

Trying to fix a furnace yourself is great, but experimenting can be dangerous and cause more problems than you already have.

7. Know When to Call an HVAC Professional

Beyond the items we've discussed, there are many other things that could be wrong with your furnace. I have identified the simple fixes above, and if you still haven't diagnosed the problem, messing around any more could compound both the problem and the expense. Although you may not want to, you should consider calling a professional HVAC technician.

Please understand that any time you work with a furnace, there is a certain amount of danger that requires your attention and care. Never perform work on a furnace if you're not sure of what you're getting into and that you've taken all the safety precautions that you can with gas, electric, and sharp edges.

I hope we were able to solve your problem, but if not, at least you will have a much better understanding of your furnace, which will help you with your upcoming service call. You'll be a very informed customer when they come knocking.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

Question:
I woke up this morning to a cold house. I went to the furnace, turned it off checked the filter, and it was fine. I took the cover off to take a look at the igniter as I turned the power back on. The fan started, the igniter activated and I heard the gas valve open, then nothing. The igniter shut off and it cycled two more times. The furnace is a Goodman and it threw an error code (1). What could be the issue?
Answer:
Assuming the gas is on, I would be initially checking the pressure switch but there are a lot of possibilities based on the information without running some tests.
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Question:
After the furnace has been idle, it frequently (75%) will not provide heat when the thermostat places an order. What happens when it doesn’t work: - the thermostat places the order - the fan starts - the surface ignitor begins to glow, then goes dark - the fan continues. The remedy that works 100% of the time: - turn off the switch on the outside of the furnace - wait 30-60 seconds - turn on switch - furnace works fine until the next long idle period. Ideas?
Answer:
It sounds a lot like a pressure switch. See if this helps. https://dengarden.com/appliances/Everything-You-Ne...
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Question:
My Comformaker furnace is showing a four flash code. I replaced the limit switch. Why is it not igniting?
Answer:
I do not know what the four flash code means as the codes vary per brand. The pressure switch just confirms that the inducer is venting properly. Just because the flash code (if it said pressure switch) says pressure switch it doesn't mean the switch is bad, just that it is why it won't let the furnace light. So, it could be bad (which seems it wasn't since you replaced it) or it could be a blockage in the chimney, the motor is bad or going bad, and a couple other less likely possibilities. Electrical testing the circuit would help if you know how to use a voltmeter and run these tests to help confirm what exactly the problem is.
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Question:
We have a Bryant furnace/AC combo unit. In the past 3 weeks we have had the inducer motor replaced, everything cleaned and checked inside, and the circuit board replaced. The pilot light will still not stay lit and it blows cold air. Any suggestions?
Answer:
If this is a standing pilot unit, then I would say perhaps the thermocouple though standing pilots are only on older furnaces now. Newer furnaces do not have those anymore.
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Question:
My furnace recently started blowing cold air at random times. If you listen when the thermostat kicks the heat on, it will try to light three times; then if it does not, it blows cold air. Other times, it lights on the first try and is fine. It seems to not light correctly at night. Any suggestions?
Answer:
Without being there, I'd say you have a dirty flame sensor or intermittent issue with the pressure switch. The blower coming on after three attempts is a default when going into lockout.
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Question:
How do I know if the gas valve is on or not? Does the switch have to be straight up and down or sideways?
Answer:
If the valve handle is “in line” with the piping, then it is open/on. If it is turned sideways in the direction of the pipe it is closed/off.
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Question:
I have a Nordyne m1mb 070a aw gas furnace. Everything on my thermostat is working fine. My furnace is switched to on gas is on now the red light on the board is not on the wires on the furnace are hot as they sparked white hot but no red light it isn't turning on, or even trying to run. What happened?
Answer:
this could be the transformer, fuse, board, or all 3.
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Question:
If the circuit board is bad, would the furnace work at all? My furnace works fine sometimes, but a couple times a day, usually in the middle of the night, it blows out cold air only. Two techs have told me circuit board is fine, but the 3rd tech said circuit board is bad. Please Help!!
Answer:
A board can cause weird things since it's in control. When it goes the furnaces can work intermittently. I'm not diagnosing your problem, however. Just answering your question that yes, a bad board may still allow the furnace to run intermittently.
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Question:
With the maintenance cover open, I push the plunger button to allow the unit to cycle. Just after the LED illuminates, I see a small spark or quick glow on the control board. I would think that is or soon will be an issue. What are your thoughts?
Answer:
I'd say you're correct. There shouldn't be any sparking on the board and boards just get replaced, not repaired.
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Question:
I have a Goodman model #GOS80703AMA furnace. It's been through 4 IFC boards in about 8 years. Each for exactly the same faulty connection problem at the board. This makes me believe there is an open limit switch. I can make it run if I hold pressure just right on the plug. Any ideas? Thanks
Answer:
Well I'm inclined to think the pressure switch is bad and would try replacing it since it should be cheap.
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Question:
Every 2 weeks or so, my furnace will not kick on. The heating and air guy has replaced 2 thermostats, control circuit board, cleaned the flame sensor & limits, checked flue pipe and checked for flame rollout. I usually just have to go to the thermostat and make it go down under the temperature and put it back up for it to kick on. Yesterday it would kick on but it wouldn't kick on to the heating cycle. It would kick back off. So I just left it off all night. What could it be?
Answer:
I don't see you mention a pressure switch... it kind of sounds like it could be on its way out. These are one of the few parts that can slowly die whereas others usually either work or don't.
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Question:
I have an Intertherm autoignition furnace. I just bought the house and the a/c works, but not the heater. So I got a new pressure switch but still have no ignition. So I dug a little deeper and found a really bad ignitor, like burnt ceramic and melted stuff. I replaced that and still no heat. I can hear a click like a valve must be opening, and I can smell gas when it does, but the ignitor doesn't seem to get hot. I'm at a loss. The control board has a steady red LED meaning that the control is ok. How should I further troubleshoot my furnace's ignition problem?
Answer:
Well I was going to say control board and though it says it’s ok if it’s not, it could say anything. Test for voltage on the ignition wire to be sure.
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Question:
My furnace was working fine until this weekend. I turned on the thermostat, called for heat. Nothing! I reset it manually from the breaker box. Nothing! I just replaced the filter. When I call for heat, the blower motor kicks on, the igniter will NOT light. I tested the igniter, it glowed. I've already cleaned the flame sensor. What on earth could be preventing the unit from working?
Answer:
There should be a light on the circuit board blinking to tell you roughly where to look for the problem. Since the blower is kicking on immediately, something has failed. Maybe the board or inducer motor since it’s happening before the unit even tries to light.
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Question:
How do you know when your circuit board on a forced air HVAC unit is defective?
Answer:
Typically you'll see bad power or no power going from point A to point B. However, there are a lot of A's and B's, so you really need to know what you're testing. There are some "flags" that can give you the idea it's bad but electrical testing is the only true way.
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Question:
If my furnace pressure switch is working but there is still no spark, what then?
Answer:
If it’s a spark missing then it would likely be the spark igniter or the spark module. Testing would need to be done to determine this unless you just roll the dice and replace one then the other or both. Keep in mind I cannot guarantee that but these are common failures that have this symptom.
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Question:
The furnace fires up and then shuts down. Based on your article, the flame sensor is bad. I replaced the sensor, and now the flash code is telling me that there is an ignitor problem. Why does the ignitor light the fire but still turns off?
Answer:
Perhaps the board is the issue, but I can’t tell without running tests.
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Question:
I have a Luxaire furnace and when it works it works fine, but every other day I have to reset it because the ignitor will not ignite. I have replaced it already and it did it again last night. I get a little flame icon on my furnace. What could be causing a brand new ignitor to only occasionally not ignite?
Answer:
The igniter is powered by the board so that is one possibility but if it's glowing and just not getting gas to light then that's a different issue and has a few possibilities.
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Question:
My furnace igniter glows hot, but the flames fail to ignite. However, if I reset the power switch the furnace lights and heats the house properly for one cycle. Would a failed pressure switch allow this one cycle to work?
Answer:
Yes, it could very well be. Pressure switches can be “finicky” and don’t always just go out. They can act funny and then die sometimes.
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Question:
I have checked the flue from the roof with a garden hose to the crawl space, and it is clear. I also replaced the pressure switch with the original manufacturer part. The manometer closes the switch at .68 IWC and opens at .58 but here is my question, why am I not getting 24 volts once my pressure switch opens and how do I get the needed supply of 24 volts?
Answer:
You won’t get 24v if it’s not calling for heat. You’d have to tap into the low side of the transformer I suppose but I’d rather you test for resistance.
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Question:
My electric bill doubled this month. My furnace will kick in and get it to the desired temperature and it will click and stop blowing hot air but it is still blowing. Then, it will click again when it needs to heat back up. Do you have a suggestion?
Answer:
It seems as if the fan is running full time either because you have it set to “fan” or there is a short causing this. That said, I run mine full time and it doesn’t have much impact at all on my bill so perhaps your motor is going bad and drawing high amperage.
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Question:
My furnace after summer failed to come on. My professional said it was the circuit and control board. He replaced it but the furnace did not fire up. It just hummed. Do you have any thoughts?
Answer:
While I can’t say what’s wrong, I can say I’d call the professional back and make him fix it properly.
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Question:
I have a furnace with the spark ignitor and the pilot does not ignite. If I manually light (light paper and put in front of the pilot) the burners will light and the furnace will run for 20 minutes and then shut off before reaching the set temperature. If I just try to run the furnace without manual interference, the inducer comes on, I hear a click, but I don't see a spark from the ignitor or hear multiple clicks. Is the pilot module likely the issue and if so, how do you test the module?
Answer:
You test to see that the spark igniter is getting proper voltage from the board. If not, it's the board and if yes, it's the sparker itself.
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Question:
Just had to replace ignitor in my furnace. The ignitor has a harness, but the furnace itself doesn't. Can I connect it without the harness and splice the wires? If so, which wires connect with which? The furnace has a black and white wire but the ignitor itself both wires are white
Answer:
Yes. You can wire nut those together, and it won’t matter which wire is which in this case. Just don’t connect both to one or all together. (I probably shouldn’t have to say that, but you’d be surprised what I’ve seen people do.)
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Question:
My A/C outdoor unit is functioning and sending cold air to the furnace, but the indoor blower motor does not get power. The fuse is ok, and the light is on solid. What else should I check?
Answer:
There are a lot of possibilities. I've written some other articles that might help you narrow it down. It's strange if it worked for the heating and everything else seems to be running, but not the fan for the AC. It could be a bad connection, or a problem with the circuit board or stat.
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Question:
I have a Lenox forced gas A/C / Furnace the fan kicks on but only blows cold air. We had it checked out, and they said it was the ignition control module because it wasn't lighting up. We replaced this piece, and it's still not glowing. What else could the problem be?
Answer:
The igniter is what I'd start with.
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Question:
I have a 2009 Rheem Upflow High Efficiency 2-Stage Gas Furnace. It is a part of the GRM Series, model # RGRM-07EAMES. The blower motor stopped working. It has a GE ECM 2.3 Motor, Repl. # 51-24374-28. All wires good, no shorts or broken wires. The fuse on the main board is OK. I took the module off the blower motor. The 2-amp fuse "run" on the circuit board blew. What would cause this? Would it be another component on the motor module? Can I solder a 2.0 amp fuse link where the printed circuit board run was?
Answer:
I can’t honestly say because it’s not a repair I’ve ever attempted or been asked to do. One thing I can say is that if the unit was registered, then you should still be under warranty. Before going through all that I would get on Rheem's warranty site and run the serial number. A new module would be best to not only address the fuse but the underlying issue that caused it because I also know that a large majority of the motors never fail, but the modules do.
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Question:
My furnace doesn't seem to be getting any gas. I don't smell it at all. What could be the cause of this?
Answer:
You really shouldn't be able to smell gas in your furnace. If you mean the main line coming in then the problem is outside of the furnace.
Helpful 1
Question:
I have a Miller Furnace model # M7RL 072A AW. The inducer motor runs excessively, the interior cabinet is very warm, and the furnace won't ignite. Would the temperature sensor not allow ignition if the temperature in the control board area is hotter than usual?
Answer:
If the high limit switch has been tripped then yes, the blower would run until the temperature drops to the allowable range and then try to fire again.
Helpful 1
Question:
I have a 2003 Lennox gas fired, forced air furnace. The gas fires up but the blower only works sometimes. The HVAC company says I need a new furnace, or new motherboard and air pressure switch. My son did the diagnostic test from the manual and can’t see that I need anything more than new relay. What is your opinion, please?
Answer:
I don't know what a 2003 Lennox design is but if the testing shows a fan relay is bad then I would definitely try it before making a larger purchase since they aren't that expensive.
Helpful 1
Question:
I have a Payne furnace which is no longer firing. The igniter relay is fine and the ignitor is glowing bright orange, but I get no voltage at the gas thermocouple solenoids, expecting 24vac but nothing, so no gas flows and no fire. Am I looking at a main board failure for my furnace or is there a sensor that might prevent the board from putting out voltage?
Answer:
I would take a look at the pressure switch first. It can cause this type of behavior.
Helpful 1
Question:
I had replaced the ignitor switch in the furnace in the winter. Now, we are using the A/C and the blower is continually running in OFF/ON/Auto and the electronic ignitor is trying to light. I thought the blower problem was a Relay/Limit Switch, but would that also cause the ignitor to spark constantly?
Answer:
There has to be something wrong with the wiring or the circuit board is shot. There is no other way a furnace would try to fire when in a/c mode. The fact that the blower is running full time regardless of mode tells me it's more likely the board.
Helpful 1
Question:
My unit turns on, but the blower fan will not start unless I start it by hand then it does fine until the furnace cuts off then starts up again. Then I have to give the fan a hand again, could it be that the fan motor is dirty and just needs cleaning?
Answer:
I hope you're using something besides your hand to do that, but it sounds like the capacitor if that's what's happening. Most often mounted to the front of the fan housing.
Helpful 1
Question:
Our furnace will run, and after a few minutes the thermostat shuts off, comes back on, and a few minutes the furnace will shut off and then back on. My thermostat is connected to the C on the mainland, so I know the circuit board is restarting. Top of core looks fine, but I can't figure out if my secondary is cracked? It was installed in 2001.
Answer:
I would need to run some tests to figure out why your unit is kicking off and on or witness what part of the cycle it's stopping. As for the heat exchanger, you'd probably need to have someone run a smoke or CO detection test and you're likely still to be under a parts warranty. Get a CO detector to be safe.
Helpful 1
Question:
On a carrier furnace, what does four green flashing lights and two yellow flashing lights indicate?
Answer:
There should be a code chart on one of the furnace doors or in your install manual that tell you what these mean. Not all brands and units are the same so I'm sorry, I do not know off hand what it's trying to tell you.
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Question:
Why has the furnace started making a knocking noise and smelling like a hair dryer?
Answer:
My initial thought is the blower motor or fan wheel. I would visually inspect that area to make sure nothing got sucked into the fan, and that the fan wheel isn't broken/wobbling/rubbing. Be sure you turn the power to the unit off before putting your hands in there.
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Question:
I have a Gibson (Nordyne) AC/Heat unit model: KG7/TC120D-45D that fails with a blinking green light. The unit has been installed for about six years and began failing last April. I replaced the two pressure switches on the left side (part #632516) the problem went away until this week (November). If I leave the upper access cover (the one over the control board and switch area) off, it stops failing. Do you have any tips?
Answer:
Taking the door off and the unit functioning as a result typically means the fan isn’t getting enough return air meaning it’s tripping the high limit which has nothing to do with the pressure switch. Those should be unaffected by the door being off. So while I can’t say exactly what issue your furnace is having, this is typically what I’ve seen. Check your filter, return duct for blockage, maybe the fan in that it’s not running slow or noisily.
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Question:
You mentioned that a common furnace problem is that the pressure could be bad. What is a pressure switch, and where is it located?
Answer:
The pressure switch is typically a round disc-shaped part with a hose connecting it to the inducer fan. When the fan kicks on, it creates suction if working properly. That suction pulls in the switch making an electrical connection that allows the next step in the cycle to commence.
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Question:
Why does my heater blow cold air?
Answer:
Something has failed and the blower is kicking on as a default, most likely.
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Question:
Are the glow plugs the same as the ignitor? There's no clicking sound, and flames won't ignite.
Answer:
You may have an issue with the pressure switch. See if this article can help to see. https://hubpages.com/home-improvement/Everything-Y...
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Question:
I added an AC to a furnace two months ago, but now the heat isn't working. What could the problem be?
Answer:
There could be a lot of things but assuming the furnace worked fine before and the a/c worked properly and no other changes were made, I don’t know how to explain it. A blockage would’ve affected the A/C too. Perhaps they changed the fan speed setting when they put it in.
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Question:
An HVAC technician was out to inspect and tuneup my Bryant 2 stage heat pump with furnace propane backup. He left, and now 2 stage only works on high. What wrong?
Answer:
He would’ve had to change wiring or a setting to make that happen I believe. If your conditions are as hot as it is here then it’s also possible there is an outdoor temperature sensor and that it know it has to run in 2nd stage to keep up with the temperature delta. You may want to give it a day or two but if it seems something is wrong then make them come back. That said, if nothing is wrong they may charge you. Perhaps a call explaining your concern is something they’ll address over the phone with you.
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Question:
Why is my blower running when the T-stat is off?
Answer:
If it’s all off, then you must have a short in the stat wire.
Helpful 1
Question:
I jumped the pressure switch because the tube keeps filling up with moisture. What would cause this?
Answer:
Improper venting is the most common cause of excess moisture. I can't say what may be improper, but that is usually the case.
Helpful 1
Question:
I have a Gilbert furnace and the inducer runs. The igniter glows brightly. The gas valve did not turn on to produce a flame when it is cold outside, like zero. Does the flame sensor detect the hot surface igniter is hot? Or does something else turn on the gas valve?
Answer:
Great breakdown of info! So all I can do is narrow it down but this is how I'd proceed. First suspect, pressure switch. You can test with a voltmeter to make sure it's closing once igniter begins to glow. Or by jumping it out and see if it fires then. If not, check for flue blockage and listen to inducer to see if sounding slow or something in there. If not, could be gas valve. Test that voltage is coming to it. If not, I'd suspect the board. Keep in mind this is just a process, not a diagnosis. A voltmeter and knowledge of what you're doing with it is important. BE SAFE! Don't do anything you're not comfortable with.
Helpful 1
Question:
My furnace will come on and heat to programmed temp, but after the first time, will not call for heat again to maintain temp. Manually shutting off the thermostat and turning back on gets the furnace to turn on and blow hot air again. I have tried a new thermostat, same issue. Any ideas?
Answer:
It depends on how low it's going. The thermostat will often have an "anticipator" that purposely lets the thermostat drop a degree or 2 below set point so that it isn't constantly kicking off/on.
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Question:
Well, I decided to buy a new furnace because mine is from 2001 and the mainboard is gone. Trying to decide between the Lennox for 5400 or aire flo for 4200 installed by an HVAC company. The 3rd option is a Concorde unit installed by a private guy for 3800 bucks but he's not tagging it.... between the 2 options the sales guy showed me what would be wiser?
Answer:
I'm not a big "brand guy". I recommend you go with whoever will do the best job installing the unit. Regardless of brand, if it's not installed properly, it won't work properly. At least not for long. I assume all these units have the same efficiency and features so if it just brand name as the difference, focus on the installer.
Helpful
Question:
Can getting a new roof make the furnace not start?
Answer:
Only if they stuffed something in the chimney (flue) or intake (if high efficiency furnace). Otherwise no.
Helpful
Question:
The light won't blink and it says to change the control. What control is it talking about?
Answer:
In my world "control" would be the circuit board (control board) or ignition module (ignition control).
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Question:
My furnace stopped running. The blower motor is trying to start (slowly). No flame or ignition. Would it be the capacitor?
Answer:
The furnace should cycle properly to the point of blower coming on before failure so if the capacitor is bad, I don't think it's the only issue.
Helpful
Question:
Does a furnace with a bad motherboard cause other electrical issues such as flickering lights?
Answer:
No. It should have nothing to do with the lights in the house.
Helpful
Question:
My flashing light sensor says I have an inducer motor fault. What can I do?
Answer:
Next would be to run electrical tests to see if the actual motor is the problem or the pressure switch or perhaps the board. If it is, in fact, the inducer motor you'd have to replace that whole assembly. Rather easy to do but the cost is dependent on what you have.
Helpful
Question:
The furnace will not turn on at the desired temp and drops 5-degrees or more before going on?
Answer:
I'd guess that your thermostat may be the issue.
Helpful
Question:
My situation is that it won't come on until I touch/pull at certain wires and then it will run as long as I'm holding on but when I let go, it shuts off and it'll show error codes (blinking led). Is this just the control module?
Answer:
It could be or it could be the harness/connector itself. These pins/connectors will sometimes get loose and cause this type of issue.
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Question:
The vent pipe from my furnace to the chimney is to hot to touch. Isn't this a lot of waste?
Answer:
That is efficiency. The flue is to be hot but it is the amount of wasted heat that determines how efficient you are. 80% efficient means 20% waste. Etc...
Helpful
Question:
The red light on my furnace stays solid? Any suggestions?
Answer:
You should have a chart explaining the red light codes. In my experience, a solid red means the unit has power and is awaiting command.
Helpful
Question:
I have a Lennox lag series rooftop unit. There is no power to the gas valve. Could this be a faulty ignition module?
Answer:
It's likely but hard to say without running electrical tests for sure.
Helpful
Question:
I have an Ultra SX 80. It seems like everything is working except the main blower fan. It makes a humming noise and does not turn on even when the thermostat is set to on. Any thoughts? Board or motor maybe?
Answer:
If the furnace is going through its processes and then after lit, the fan doesn't come on, and the unit shuts down again it would likely be the blower which if changed should change the capacitor on the blower too. That's really the best I can offer you from here.
Helpful
Question:
I have an electric furnace. The coils heat up, but the blower doesn't come on. I can hear a clicking when it tries to come one. What could be the problem?
Answer:
Possibly your blower motor is bad. That's what I would be checking first.
Helpful
Question:
Why does the flame stay on when a furnace is off?
Answer:
A pilot light (little flame) always stays on if that's what you see. If it's the burners, then I've only seen that on pretty old units, but there isn't enough info for me to offer any more info other than it shouldn't do that so you should have it looked at.
Helpful
Question:
Why does my thermostat read RECOVERY every so often?
Answer:
This is programmable? It means it's getting back to an "occupied" set point. I.e., You were gone, so program for one temp and then another for your return...it's in "recovery".
Helpful
Question:
The flame lights or a minute then goes off. I replaced the flame sensor and ignitor but it still does the same thing. This is with honeywell smart valve. Any suggestions on what might cause this?
Answer:
Well since you've replaced the sensor I'd check the hi limit or the board. Perhaps one of those are the culprit. I would need to run electrical tests to confirm though.
Helpful
Question:
Why is the zone 2 light red on my Honeywell furnace? Is it not working?
Answer:
I'm sorry but I can't answer this because Honeywell doesn't make furnaces and this would be in regard to what I believe is a zoning system which I cannot troubleshoot over the net. Sorry I can't help on this one.
Helpful
Question:
The blower will run, but no burners come on?
Answer:
The blower runs at a default often when a failure occurs. Unfortunately, it doesn't tell me anything about why it failed. Sorry, I can't really offer any advice based on this information.
Helpful
Question:
My furnace will stay lit for somewhere between 30 minutes and a couple hours but then will shut off and show a lockout code. Should I clean the flame sensor?
Answer:
While it doesn't hurt to try cleaning the sensor, I'm not sure that's your issue. Usually a unit fails, for whatever reason, 3 times before it locks out so I would try and see what's happening prior to that. That might give you a better idea of where in the cycle the breakdown is occurring.
Helpful
Question:
Why would I have voltage running through my gas lines on the furnace?
Answer:
I'm not really familiar with this though I've heard of scenarios where it's happened. You may want to try a couple of searches to get info about this or contact a plumber. If you're getting shocked, you need someone to address it and likely should be a professional but probably a plumber, not a heating contractor.
Helpful
Question:
My furnace will only run when the thermostat is set to 'ON' while in the auto setting the burners do not ignite and the blower will not run, but as soon as I change that setting everything works. Any thoughts on where I should pinpoint this down to?
Answer:
It sounds perhaps like the low voltage wiring isn't correct. Not something that would just start out of nowhere though unless there is a short in the circuit. This is my guess from afar.
Helpful
Question:
My electric furnace continues to run after the cycle, I've replaced the thermostat, board, and now the sequencer. After it cools down, I turn the power on, and it won't kick on until the thermostat tells it to, but will not shut off. What could be the problem?
Answer:
The blower is supposed to run after a cycle until the fan limit tells the motor it's cool enough to turn off. If that's not happening then likely you have a bad fan limit or problem in the board. Keep in mind these are just educated guesses without running any tests to confirm.
Helpful
Question:
What does it mean when the light goes out then a flash?
Answer:
It likely means you don't have power to the unit or perhaps blew the board fuse.
Helpful
Question:
What would cause my furnace elements to keep burning after the fan shuts off?
Answer:
A weak, slow closing gas valve could cause this. I assume this is an older unit and I suggest a service maintenance to be sure.
Helpful
Question:
The ignitor on my furnace lights up bright but does not seem to be lighting. What could be the problem?
Answer:
If your furnace is blinking a code, compare it to the chart either in the user's manual (you may be able to find that online) or, there is probably a chart on the furnace to tell you what the different codes mean.
Helpful
Question:
I think I need to switch out the capacitor to my AC unit. Where is it located?
Answer:
I think this article will help you find and see what is entailed in the repair.

https://dengarden.com/appliances/How-to-Change-an-...
Helpful
Question:
Is there something on the circuit board that controls the volume air needed to cool my home?
Answer:
There are fan speed terminals that determine what speed the fan will run at during heating, cooling, and fan modes. If this wasn't an issue before then, it's not likely the issue. Check your filter and perhaps your evaporator coil (if you can) as they could be plugged and blocking air.
Helpful
Question:
I'm installing a Nest thermostat, but I'm not getting sufficient voltage from the furnace panel to run it. What could be causing inadequate voltage from the circuit board?
Answer:
When it comes to circuit boards in furnaces, there isn't really any repairing them, and you may have a bad terminal which would require a new board. Make sure to check that the stat only needs 24v.
Helpful
Question:
There is no power light or any light at all coming from my furnace, but I hear a sound like it wants to come on, but nothing is happening? What can I do?
Answer:
It sounds like you have power to it, but I'd check that first. The hum is likely a transformer if it's a rather soft hum, (not a motor stick hum). I'd check the fuse on the board. It is probably a 3 amp car type fuse. If that is good, I'd check power to and from the transformer, (120v in 24v out). If all that is correct, then I'd be leaning towards the board being bad, but I can't really confirm that without being there.
Helpful
Question:
I turned the heater gas knob from "on" to "off" a couple of weeks ago. (There's also a "pilot" option on the knob.) Now, when I turn it back on, the heater doesn't work. How can I start the heater again?
Answer:
It sounds like you need to light the pilot. Turn to the pilot. Get a match or lighter by the pilot, then press down on the button. Once lit, hold the button down for another 30-45 seconds. Next, let go and turn the switch to "on." I also wrote an article about this in regards to water if you need more insight. They are basically the same process.
Helpful
Question:
The status light on my Intertherm furnace is not coming on. I have power going to the circuit board, but there seems to be no power going to the blower motor or anywhere else. What could be the problem?
Answer:
It may have a blown transformer, or perhaps the little three-amp fuse that is often on the board could have blown. You can easily see if that fuse is blown by just removing it and looking at it.
Helpful
Question:
We have a Nest Thermostat. A couple of weeks ago our Nest told us it had a low battery and then shut off. When we pulled it off the wall we found out I could charge it with my phone charger. Once it's charged, and we put it back on the wall it works, but we have to keep charging it because for some reason it's not getting power. Could this be a fuse problem?
Answer:
No. It sounds like they never ran the wire to power the stat from the furnace. That would be my guess.
Helpful
Question:
None of my status lights are on. My heat isn't working either. Is it the blower?
Answer:
It sounds like you don’t have power to the unit or perhaps blew a fuse on the circuit board. Check your breaker and service switch as well as the little 3-amp fuse on the board.
Helpful
Question:
My blower will shut off at the end of a heat cycle then run another 3 seconds. What can cause my furnace's blower to shut off?
Answer:
The blower always continues to run until the temperature in the exchanger is depleted and satisfies the limit. This is normal.
Helpful
Question:
The heater runs, but I have to flip a switch on top of the junction box before it will come on. When it comes on, it makes a clicking noise that sounds like it is coming from inside the junction box, but it turns off fine when it reaches the desired temperature. What could be the problem?
Answer:
While I can’t say what the issue is for certain, you’re probably resetting the board by flipping the switch.
Helpful
Question:
There is power going to the furnace, but the Indicator light not working. What does this mean?
Answer:
Likely it’s a bad transformer or board but that can’t be for certain without testing.
Helpful
Question:
I need a new ignitor for my furnace, but I haven't had it fixed yet. Can I run the air conditioner with furnace needing this part? I assume it would be ok.
Answer:
Yes, you are fine running the A/C.
Helpful
Question:
the igniter burns as soon as it runs. I replaced it twice, but the same problem occurs. What should I do?
Answer:
If the igniter is getting constant power when there is a call for heat, and that's what's burning it up, then the circuit board is likely bad or the igniter is wired in wrong. But that is typically difficult to do.
Helpful
Question:
Why would a thermocouple need to be replaced if the furnace pilot ignites?
Answer:
The thermocouple confirms that the pilot is and stays lit. If the pilot lights but won't stay on, it may be because the thermocouple is bad and telling the unit it isn't lit, thus shutting down the valve.
Helpful
Question:
My Goodman gas furnace blows a fuse as soon as the burner lights. What would cause this?
Answer:
I'm not sure what's happening there, but I can say that something is shorting or drawing too much amperage. Electrical testing would be required to determine the source.
Helpful
Question:
My furnace gives the reverse polarity code, but I can tell that the polarity is not reversed. What is wrong?
Answer:
If you are confident that your wiring is correct then the usual suspect is the circuit board.
Helpful
Question:
I have an Intertherm furnace that's in a mobile home. I've replaced the thermostat (which only has a red and white wire connected to it) and the flame sensor. I light the pilot light, and my furnace will come on for a few minutes, but then the flame dies. What else could be the cause of this?
Answer:
Perhaps the fan/high limit? Dirty filter or blocked airflow? These are a couple of common suspects.
Helpful
Question:
Why does my breaker keep kicking on my electric furnace? My landlord doesn't know how to fix it, what should I do?
Answer:
Your landlord should be calling in a technician to repair that. There's likely a short or something overheating causing it to trip unless the breaker is just bad. Either way, it needs to be tended to.
Helpful
Question:
I have an HE gas Lennox 92AF1UH that seems to be leaking from the exhaust moisture collector (not the technical term). When we opened it up initially the filter was completely waterlogged & moldy. We blew out the hoses and dried everything out, but there is still moisture accumulating after its run. The unit is five years old. Any thoughts?
Answer:
While water is difficult to track the source/problem via the internet, I can say that a 5-year-old Lennox may well still have a parts warranty so if you get a tech you might only have to pay labor. Your heat exchanger definitely has a warranty if it's the issue.
Helpful
Question:
When I turn the furnace on, it starts the blower fan and the inducer fan and slowly the inducer fan slows to a stop by the blower fan keeps running. While all this is going on the hot surface ignition switch never stars to glow and it never fires up what could be causing this?
Answer:
I believe it's the pressure switch. Check that the chimney isn't blocked or the inducer seeming weak or grinding but if those seem ok I believe that's the problem based on what you've told me.
Helpful
Question:
My furnace has power through the primary power source. The fuse is good, but the indicator light still isn't working. Will a bad thermostat cause this, or do I have something else going on?
Answer:
You could check the power at the transformer for 24v/120v just in case, but it may indicate the board is bad. Is everything else working? There is a slim chance that the bulb is bad, but it's happened before.
Helpful
Question:
What could be the reason for a thermostat to stop working for a month and then resume working?
Answer:
Are you sure that was the problem? It could be the batteries or the fact that you may have changed something else.
Helpful
Question:
I have a carrier ts unit. In the middle of the night, it stopped. I have a yellow light flashing, not red. If I turn the fan on, it runs. So it is getting power. What might be the issue?
Answer:
It may have been a fluke if it's not a regular occurrence. That does happen sometimes.
Helpful
Question:
I smelled burning wires, and then my ten-year-old trane single cycle furnace stopped working. An inducer was replaced, but the problem remains. What could have caused the burning wire smell?
Answer:
A transformer can smell pretty good when it burns up. I would still recommend that a technician check that for you. It's probably not a bad idea to shut down the power to the unit if it's not running anyway.
Helpful
Question:
Our furnace has turned on, but wouldn't ignite twice in the past week. When we reset it manually in the breaker box, it works correctly. Any ideas what could be wrong?
Answer:
There are a couple of parts in a furnace that can either be going bad, or just doing what they were supposed to do due to something happening at that moment. A dirty flame sensor can be intermittent, so cleaning it is essential, or it will become more frequent. Perhaps the pressure switch is going bad, or maybe lousy timing sent a draft down or through the flue, which can trick the switch into thinking it's not getting the right pressure. If that happens more than a couple times in a week (assuming no heavy winds), you may want to replace it before it goes all together. These are common occurrences that are easy to check or fix yourself.
Helpful
Question:
The thermostat panel went blank while the furnace was on. Now, it is blowing cold air. How do I turn it off, and what can it be?
Answer:
There should be a service switch near or on the unit that you can turn off, or at the breaker. It sounds like a short, but I'm not sure where.
Helpful
Question:
I have a very old 1986 furnace and it is not burning well. It produces a yellow flame, a lot of soot, and strong smell that feels like it's lacking oxygen to burn. What would it be?
Answer:
Not to seem curt, but it would be time to consider a new furnace. I can't say for sure without testing, but it sounds like you have a bad heat exchanger and that can be dangerous if it, in fact, is what is the problem.
Helpful

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Comments

Robert Estrada
5 weeks ago
I have a old ruud unit 1994 and i the light that says power is not on. I tried looking for fuse but it dosen't seem to have one. I have no power going to the circuit board. It was running earlier but sounded like it was struggling then turned off. I checked breaker and nothing.
Je
3 months ago
I have a new furnace and the inducer will kick on and off while in cool mode
Chuck
4 months ago
My problem is when I went to start.my air for the first time this season,not sure what happened nut now no furnce and the air did not come on, would appreciate and input, Thanks..
Brett Winter
4 months ago
I recently added an a/c to my gas furnace. When I hooked up the extra wires for the a/c, the furnace would no longer work for heating. The small fan kicks in and the furnace lights but then the burner trips and the big furnace fan starts up. If I disconnect the a/c wires the furnace works fine again
James
4 months ago
Thermostat seems to be working fine. Turn heater on but no glow on the ignitor. Checked ignitor seems ok. Fan constantly runs but nothing else happens. I'm stumped
Gioia Smith
4 months ago
I'm trying to find out how to thermostat is staying on 88° red lights are flashing on the side how do I get is turned off
Sherri
6 months ago
Thank you for posting this and allowing us in on your valuable experiences and expertise. I think it has helped us fix the problem on a Sunday as well as calling a local HVAC guy! We appreciate your help via Internet!! God bless!
Eric L
7 months ago
Hi, Dan. Thanks for your straight-forward troubleshooting tips - they are VERY helpful. I've run into an issue on our furnace (model Nordyne G6RA120C-20) where the red light says the "pressure switch stuck closed", not open. There are no blockages in our flu, I replaced the filter (just in case), cleaned the flame sensor (for good measure), and removed the pressure switch to see if it was obstructed or visibly stuck in some way (9300-V-0.55-DEACT-N/O-VS). I could hear the mechanism inside moving, hooked it back up, and it worked once for about 10 minutes. Now it's back to the same routine - vent motor comes on, click, ignitor comes on, click, ignitor goes off, red light flashes the "pressure sensor stuck closed" error, and tries again a few times. We have a very dry basement and I don't think it has to do with water, but I'm guessing the pressure switch is bad. Any thoughts on a "stuck closed" error like this?
Ray
8 months ago
I have a goodman up draft air & heat sys , what is going on is I set the temp like at 72 the heater comes on and runs for about 10 to 13 min and the temp on the thermostat reads 72 the sys shuts off then I can hear the vent blower come on as soon as everything had turned off it runs and then the burners come back on again and everything runs again for about 10 min.Is this normal I don't think it dose it every time like this but a few times it dose and even it cycles 3 times and runs for about 10 min. I have a nest thermostat dose it have something to do with the way that type of thermostat acts ?
Nick P
9 months ago
I have a variable speed 2 stage AS furnace. Lately I haven't heard the furnace kick into the 2nd stage. I tried to test it by setting the thermostat higher about 15 degrees and waited almost a half hour and it never kicked into stage 2. It remained on low flame stage 1. Can there be something preventing the furnace from kicking into the 2nd Stage?
AUTHOR
Dan Reed
9 months ago
SG - While I can't say for sure without being there and testing, a blown transformer could be the issue. Since it's pretty cheap, it might be worth trying before saying it's the board. Really, you should run some tests with a voltmeter to pinpoint the problem and avoid spending money that you might not need to.
SG
9 months ago
I have a Goodman GPG1360140M41AD. It was working fine and then it blew the 3amp fuse on the control board. I checked all the wires and replaced the thermostat and replaced the wire from the thermostat to the furnace. It first began to recycle and not ignite and now it blows the fuse once it tries to ignite. So it was not the thermostat. Can a dirty flame sensor cause this issue or should I replace the control board.
AUTHOR
Dan Reed
9 months ago
Thank you Mark! I really appreciate the feedback. I take a beating sometimes from guys who feel I'm taking business away from them and well, I suppose I do, but my intent is to help those who can't afford to pay $150 or more to come clean a flame sensor. While I don't intend to take work from people my loyalty is to those who want to be warm and not at the cost of their family's Christmas when the fix is simple. I'm glad you see that. I hope this and other articles never have to come in handy for you but if they do, I'm glad I can help. Best wishes to you and yours.
Mark J
9 months ago
Dan - you, and guys like you, that post basic knowledge like this are freakin' awesome. I can't tell you how much it is appreciated from a layperson's (non-HVAC technician) perspective. I'm more that happy to get on a professional's schedule and pay them to come out and fix an issue - but your type of posting really helps to cut down on the "duh" service calls - and that is extraordinarily helpful. Thank you sir.
AUTHOR
Dan Reed
11 months ago
Rich - The pressure switch clicking when you blow/suck on the tube doesn't mean it is working properly. It does sound like that may be the issue still.
Rich A
11 months ago
Dan- My furnace is attempting to start the motor starts for about thirty seconds and shuts off. I checked the pressure sensor manually by blowing into the hose and can hear it opening. The furnace itself is giving code of "Pressure switch stuck open". It's my understanding often times you get a pressure switch error but the problem is other components. The ignitor does not appear to light whatsoever, I hear no clicking, smell no gas. As far as I can tell there is no obstructions in the flue. Any ideas? Im guessing the ignitor or flame sensor could be faulty.
AUTHOR
Dan Reed
11 months ago
Jenn - While I can't be sure of the issue it does sound like a pricey repair for a 17 year old furnace. $1000 is a nice down payment on a new unit and there is no repair I can think of that would cost that much unless it was to replace a heat exchanger (which again, I would rather do a furnace for that money) or a complete overhaul of the parts there which makes me leery that he doesn't know what the problem is so just decides to replace everything. Sadly, these "part changers" are out there. Please get a 2nd opinion.
Jenn Alb
11 months ago
We have a natural gas forced air furnace-17 years old. When I turn it on, it will light, heat, and blow hot air until it hits temperature and shuts off. When the temperature drops a degree and it should go back on and heat, it will click back on--it will click to start, you can hear it trying to go-then it does not light. The blower goes on anyway and begins to blow cold air into the house--which then makes the thermostat temp drop and it just stays on blowing cold. In order to get it to work again-I have to shut it off at the thermostat, wait at least 90 minutes or so, and then turn it back on. It heats nicely--will superheat to 78 degrees if I let it (we did this last winter to give more time with it off on cold days). HVAC told us $1000 to fix--but I think that can't be right since it still works fine--I am thinking gas valve or thermocouple. My husband is handy, rebuilt outside air conditioner, replaced anode rod in water heater, replaced fan on water heater, etc... but afraid of messing with gas esp. since my son's bedroom is in basement near furnace.
Any suggestions Does that sound like a rip-off for repair?
Thanks!
AUTHOR
Dan Reed
11 months ago
Penny - If the stat is off then you shouldn't have heat but is the fan set to "on"? That would allow the fan to just simply run like a fan and only call for heat/cool when necessary. If everything at the stat is off then you must have a short circuit somewhere in the stat wiring or control board. You may need a tech to do electrical tests to be sure.
AUTHOR
Dan Reed
11 months ago
Tracy - While I'm not sure what the cause is, it's not normal and likely not the blower's fault but something else causing it. Bad voltage, filters not being changed regularly, improper duct size starving the motor for air...there are a lot of possible causes but this is what I'd be checking for.
Penny Escalante
11 months ago
My furnace blower is running but not blowing out heat, my thermostat is off, what do I do?
Tracy G
11 months ago
Hi Dan, We have replaced the blower on our gas furnace the last two years. After it is replaced, it works fine for the rest of the winter, then when we try to use the furnace come the next winter it won't work again. This will be the third year we will have replaced the blower. Any idea why this would happen?
AUTHOR
Dan Reed
12 months ago
Brad W - Unfortunately there isn't enough information for me to help. Maybe the stat is bad? Maybe a limit? Sorry.
Brad W
12 months ago
Hi Dan, I have a forced air coleman furnace that only works when the thermostat is turned all the way up and only runs for a few minutes and only blows warm air, any ideas on the reason? thank you
AUTHOR
Dan Reed
12 months ago
Tammy - I'm betting that it's the igniter. When you here the clicking that is the spark ignition trying to light....do you see a tiny spark? If you do, then the problem is either the gas valve not opening or the gas is off or potentially there is air in the line and it may need bled out. These are just guesses of course but all things I would be looking at if I were there.
Tammy
12 months ago
I turned my furnace on for the first time since winter it worked fine in the winter but now wont turn on. I hear it clicking trying to light but doesnt. What could be wrong??
AUTHOR
Dan Reed
12 months ago
RAur - Thank you for the feedback. I think it's a little high but I'm in the "Heartland" so perhaps that's reasonable for the west coast. I wrote an article about cleaning the flame sensor...I hate to say it may have just been dirty. These usually fail or they don't...they don't usually act erratically unless they need cleaned. If they fail, they just fail. No resetting will help it. I have to clean mine every year.
RAur
12 months ago
Dan, I got my furnace fixed, but needed a tech/service call. Essentially, it came down to the flame sensor being old and needing to be replaced. It was functioning erratically with no consistency. Manually recycling the power was helping, but with that i was was getting lucky with it working at times and not at other times. The tech replaced the flame sensor. The visit and the flame sensor together cost me ~$300/-. I am not sure if that is expected. I live the pacific northwest close to Seattle.
AUTHOR
Dan Reed
12 months ago
RAur - You're welcome. I do have other articles here that may help you find where the fault is occurring if you check on my profile page, you'll see the list and can determine if one may be helpful.
AUTHOR
Dan Reed
12 months ago
Anna,
It sounds as if you have a short in the low voltage wiring, stat, or at the board. You're correct. If you have it set for a/c you shouldn't be getting any sort of call for heat.
RAur
12 months ago
Dan, thanks for your response. Your explanation makes sense, at least is logical. In terms of the fault that might be occurring, I will read a bit more to understand how i would find it. I will call for a service technician soon. When I get it resolved, i will revert back to this thread, so it might be beneficial to other readers of your site and for search engine indexing.
Anna
12 months ago
Our furnace is not on but is glowing like it wants to ignite. We have the air on so it should not even be trying to light.
AUTHOR
Dan Reed
12 months ago
RAur, I'm sorry but there are too many possibilities for me to try and narrow down based on the given information. It sounds like a fault is occurring and when you reset the power you're resetting the unit, clearing the code and the unit is running until the fault happens again and you're back to square one. What the fault is, I couldn't say without running tests or observing where the unit is shutting down in the cycle.
RAur
13 months ago
Dan, I have a nest thermostat and an older furnace. They have been working well. But now the furnace (fan or heat) is not turning on even when i set the temperature on the thermostat higher than the temperature at home. However, i am able to get the fan and heat going when i manually turn off and then turn on the furnace's electrical switch. Any thoughts? Thanks.
AUTHOR
Dan Reed
13 months ago
It sounds like you have an older furnace so there aren't too many parts there. It could be the limit or gas valve going/gone bad. I don't have enough info here to say for sure but that's what I'd be looking at.
Joe Moore
13 months ago
Hi my furnace is gas, the blower turns on in the summer to cool itself. The pilot lights and stays lit. Last winter my furnace wouldn't kick on to blow heat when my thermostat was reading a significant drop in temp. It stopped heating my house. That part hasn't worked since. Any suggestions?
AUTHOR
Dan Reed
15 months ago
Jordan beyer - If it's frying the breaker then you have an electrical issue that I am not sure what to offer as a cause. Bad wire? Bad breaker? If it's just tripping the breaker however then something in the unit is trying to draw too much power thus would short the board and then trip the breaker.
Jordan beyer
15 months ago
My circuit board keeps getting fried and the breaker not sure why any thoughts
AUTHOR
Dan Reed
16 months ago
E Thomas - It very well could be. When there are funky readings and weird activities that just don't seem to tie together for logical conclusion, the board is a pretty regular suspect.
E Thomas
16 months ago
I have an issue with m HVAC unit where the blower runs non-stop, but the AC (or the furnace) wouldn't turn on. I have made sure the thermostat is fine. On the control board, there is no voltage between R and C. The fault LED is flashing an error - Open RO Switch. I have confirmed that that the RO switch circuit is closed. Could this be a bad board?
AUTHOR
Dan Reed
16 months ago
I really can't say exactly what is happening but if it started with the new exchanger they should be fixing or at least figuring out what the issue is for you. I know propane at the wrong pressures can produce soot...perhaps it was wrong before he came but a bad exchanger had covered that up somehow? Do you know for sure that the unit was converted to LP? Units almost always come set up for gas and then need an LP conversion kit to make it work with LP which means changing out the orifice and valve spring as well as balancing the gas pressure to whatever the unit requires. If these things aren't done it will have your unit all goofed up.
Phil
16 months ago
Hi, last February I had a Heat exchanger replaced in my Miller Propane Furnace. The Furnace starts ok to where ever the thermostate is set for, The problem i am having, is the furnace is turning off and on about every minute about 10 times before staying off. The roof vent is completely covered in black soot and it is all over the roof, None of this occured before the tech changed the heat exchanger. What could be the problem?
AUTHOR
Dan Reed
16 months ago
Jerry - I cannot say for you to do that. I do not have enough info to recommend you do anything with those wires other than what they're intended for. If the problem is new then the wires are not likely the issue. Kill the breaker to make the unit stop.
jerry
16 months ago
is there any way i can unhook them to wires and run them some where else to see if the unit will shut down or not with just them two wires?the unit only has heat hook to it.
AUTHOR
Dan Reed
16 months ago
Jerry - I would imagine the green is hooked to the white terminal then those 2 wires would give you heat but they may have used a device call "add a wire" and then it may be all they needed to do the a/c when that was installed as opposed to pulling new wire. That is my guess.
jerry
16 months ago
all the wires on the honeywell digial thermostate are attached didnt hook them up myself but where it comes into the wall to th pigtail plug wiring harness there is only two wires hooked up a red and i believe a green one and all others where cut short there
AUTHOR
Dan Reed
16 months ago
Jerry - I don't believe the stat is faulty since there would be other features that would shut down the unit if it overheated and if it won't shut off then it sounds like something is mis-wired perhaps.
jerry
16 months ago
have an intertherm electric furnace hooked the power up to it set the thermastate and it was all ready running waiting for it to turn off at th temp i set it but it never shut off and the blow was still running on hight and i could see the coils still glowing red tryed everything to shut it down till i pulls the disconnect over the cartages fuses and it shut down could it be the digital thermastate being faulty?
AUTHOR
Dan Reed
16 months ago
Mrs. Klann - While I can't say what's causing this I can say that the unit should be shutting off then because if the flames run without the blower it will overheat and shutdown. If that isn't happening then I would quit running it because it could become a safety hazard. You'll need to call a technician however to find out what's the cause of this. Maybe a bad limit switch or circuit board but again, I can't say for sure based on this little bit of information and not testing the circuit.
Mrs.Klann
16 months ago
My furance flames come on-but blower wont run-unless I keep turning it on by thermostat switch??
AUTHOR
Dan Reed
17 months ago
Jesse - The answer to this all depends on your mechanical abilities. For someone who is familiar with taking things apart and putting them back together I'd say yes, it's a pretty easy task. If you're not, then it could be more hassle and lead to more cost than it's worth. If you do, don't forget to buy a new capacitor with your new motor. Best of luck!
Jesse
17 months ago
I think I have a blower that is starting to fail. Last year I could hear it making a squeaking sound but would always come on. Now it won't come on and the red light is blinking.
Is replacing a blower easy or should I get an HVAC tech?
AUTHOR
Dan Reed
17 months ago
Shawn - it could be so many things. Try and catch the code it blinks if you have that LED on your unit next time it breaks down. That would help narrow it down. If not, he really should call a tech for you.
AUTHOR
Dan Reed
18 months ago
Bill - I'm not sure what would cause both the fuse to blow and then when that's replaced still not run. Being brand new, I would certainly call the installer and ask them to come figure out and fix the issue.
Bill
18 months ago
I have a airtemp gas furnace brand new it ran all night on and off only ran 5 min then went off got up this morning nothing found a blown fuse on the board replaced fuse now ignitor glows but will not light
Mollie
18 months ago
I have an intertherm furnace, the thermostat died, changed the batteries and it works but the heater won't come on, the diagnostic light shows no problem, I turned off the switch and restarted it and nothing
AUTHOR
Dan Reed
19 months ago
Joe, Did you resolve the issue at the thermostat? Just turning it off and on may not have solved the problem. Otherwise, without more information or personally running tests, I'd have a hard time offering any sound advice.
Joe
19 months ago
I have a Lennix G27M 4–100a-2 and is turning on every 10 minutes and working for approximately eight minutes and shutting down, this is after finding out that the thermostat was not working properly and I turned off the thermostat and turned it back on, what's going on please help and thank you
AUTHOR
Dan Reed
19 months ago
If the igniter is coming on but not lighting then the gas valve isn't opening. That could be because the pressure switch isn't being satisfied or it is failing. If the igniter itself isn't working then you'd have to replace it. I've written other articles that may help you narrow down your problem and perhaps make the repair.
Kami
19 months ago
Hello, my furnace was coming on and cycling about 5or 10 sec. later. I cleaned the flame sensor and now nothing. the ignitor switch now coming on slower than usual and now the flames aren't coming on. it just shuts back down,
AUTHOR
Dan Reed
20 months ago
Perhaps a burnt out heating element? Definitely an issue though since if there's an odor like an electrical burn then something is shorting out and I'd have a tech come sooner than later.
Me
20 months ago
We have a Coleman electric furnace and it clicks like it’s turning on but it’s not actually kicking on to blow air...and it smells like something is burning when it’s trying to click on
AUTHOR
Dan Reed
20 months ago
It's possible and if so then make sure you get stainless steel burners since you have LP...they'll last longer.
Darlene Statz
20 months ago
I have a 14 year old Rheem outdoor combination propane furnace/ electric air. In the last six weeks or so, on 3 or 4 occasions when I got up in the morning my house felt cold. When I looked at the temperature on the thermostat it read 61 when it was set at 68. After I turned the heat off and on, it went back to working fine until the next incident. The repairman says it needs a new burner. This does not make sense to me. What do you think?
AUTHOR
Dan Reed
20 months ago
You should have a chart on your furnace that answers that for you. I do not have these memorized. Every brand and unit has it's own coding.
AUTHOR
Dan Reed
20 months ago
If the heat strips are working too hard then the breaker trips. For some reason the amperage is spiking and therefore it's working to hard, there's a short, or there's bad voltage. You need to know what is overloading the circuit. Probably a bad element or going bad. Too much resistance and/or heat due to bad amperage causing breaker to trip.
Rose
20 months ago
My electric furnace blows hot air for about an hour and then the furnace breaker trips and it starts to blow cold air. The house breaker is fine. It's the breaker on the furnace that keeps tripping. What could it be?
Travis
20 months ago
I have an amana furnace and its flashing a red light 6 times
Sanjay
20 months ago
SO SO Helpful!! THANK YOU
Ryan
20 months ago
Hello, my furnace is showing a 4 red light error indicating the high limit switch has opened. When the tech arrived he saw the error and tested the switch. According to his test the switch was closed (though the 4 red light error continued to show). He examined the rest of the unit and found that the capacitor was starting to fail (he replaced it). Within a half hour of his departure the 4 red light error appeared again. He believes that it is the circuit board that is the issue. Unfortunately for me he had a few other jobs to deal with before returning to me. He told me to reset the furnace (turn off and on) and remove the access panel to further reduce the heat build up. Since doing this the furnace has run fine.
My question is, does this appear to be a circuit board issue? The furnace has been professionally cleaned very recently (last week) and the filters are brand new. Any insight would be greatly appreciated.
AUTHOR
Dan Reed
20 months ago
Cj - I'm sorry but there just isn't enough here for me to offer any real sound advice.
AUTHOR
Dan Reed
20 months ago
TD - I really can't say from here. You'd have to run tests to see if that's the cause.
TD
20 months ago
My nest was not getting enough power so I used extra wire to hook common to thermostat. Then a couple days later the nest says no power from R. The furnace board has green light. I tried removing the common wire I had connected before and still no power from R. So I'm not if what I had done before caused this new problem. Anyway I took the extra wire I had used for common and connected that to R on the board and R on thermostat. Now furnace works however fan will not turn off and I have to keep recharging my nest with USB every day. Is there something wrong with the board or do you think it's the thermostat?
Cj
20 months ago
Furnace stopped working in the middle of the night, turned it off, turned it back on, nothing. Went into the garage and unplugged it, then plugged it back in while it was on and it began blowing air. Flames never came on, but the furnace continues to blow air. I removed the cover for the circuit board and checked the red light on it after turning the furnace on, the light stayed a constant red. I conducted the jumper wire test which resulted in the thermostat not being the problem. I am stumped, because I replaced almost every part on the furnace within the past 5 year's.
AUTHOR
Dan Reed
20 months ago
Jose - There is not enough information here for me to offer advice. I've written quite a few articles that you could read and see if you can narrow down the possibilities a bit.
AUTHOR
Dan Reed
20 months ago
Shirley - I can't offer any advice based on this. I would really need to physically inspect the unit for a stuck open damper, back draft, or something of that nature.
AUTHOR
Dan Reed
20 months ago
Ian - Check your filter and if the a/c coil is clogged you could be trapping too much heat in the plenum. Or the limit switch itself could be bad. These are where I would start.
Jose
21 months ago
I have a rudd force air furnace heater..thats warm up a little then wont start back again unless i switch button to off to heat again..cant find where filter is located
Shirley tait
21 months ago
I have a free standing gas fireplace .Will start is and it may work real good for a couple days .Then all of a sudden I look and blue flames are dancing all around in a swirl .I get scared and turn it off ,
Ian
21 months ago
My Lennox SLP98 modulating furnace has a red blinking decimal light when on idle mode but it gives an E250 error code( Limit switch circuit open - check why limit is tripping, overfired, low air flow) when it fires up. How do I fix this fault?
Thanks
AUTHOR
Dan Reed
21 months ago
van - Well the ignitor is good but beyond that I cannot offer any advice based on this information. Sorry.
van
21 months ago
my older carrier gas furnace ignitor glow but the won't turn on
Pamelarenae
21 months ago
Thank you so much for your help. We will contact landlord again and send letter addressing also.. Merry Christmas
AUTHOR
Dan Reed
21 months ago
Pamelarenae - I wish I could offer more help and given what you've told me I would say there is definitely an issue and the landlord MUST have someone come back if it's "black as coal" because it's not normal or healthy if it's spewing it all over the home. There would definitely be an issue that needs addressed under those circumstances and by law, the landlord must address the issue.
Pamelarenae
21 months ago
Its natural gas and he does not believe duct work has been done. He said " The unusual amount of dust that I was getting was actually not dust, it was soot that the furnace has been spewing all over the house" ..the filter is black has coal. Landlord will not send her contrator back out, plus contractor only speaks russian n sasly my friend does not know russian to ask questions. Landlord said in spring she will have ducts cleaned.. it is 2 story townhome, furnace is in basement not in same room with washer dryer if that helps.
Pamelarenae
21 months ago
Thanks for answering question above.. im waiting to gear back from friend. I know he has natural gas and has commented on how dirty everything gets in his home very fast. Ill write again after he responds to my text. My friend moved in about time furnace was put in so im not sure he will know alot about past duct work. Thank you again.
AUTHOR
Dan Reed
21 months ago
Elisabeth - I'm not sure what "steady on" means for your particular furnace. There should be a code chart that says if that means "normal operation" or a particular failure. Is it lighting? If so, then I'm not sure how you'd have cold air. I know that sometimes air can perhaps feel colder than it actually is but if there is a flame in the unit, I assure you it's not as cold as you might think unless for some reason your a/c and heat are coming on together. (Don't laugh...I've seen it before.) Sorry I can't be of much more help based on the information you've provided. There are just too many possibilities to narrow it down based on this. Thank you for reading.
Elisabeth Maynard
21 months ago
My furnace pilot light is on, red light and yellow light are steady on but I'm get nothing but cold air. I've checked the filter and everything else I can think of. My formate is a Nordyne model # M1MB 070A BW. PLEASE HELP
AUTHOR
Dan Reed
21 months ago
Dave - I answered the other question but here there are a lot of moving parts and I'm not sure what you're dealing with. If the fire is on, the air cannot be cold. If the furnace is failing, then the blower will come on as a default and then yes, you would have cold air but after 3 times, it should go into lock out. After all you've done, I would recommend having a tech look into this since it will likely require some tests that I cannot run from here. I'm sorry.
AUTHOR
Dan Reed
21 months ago
Ryu8 - Glad to hear!
Ryu8
21 months ago
Problem fixed. Thank you so much.
Dave
21 months ago
I have HVAC furnace model number KG6RC 080C-12B. I was experiencing problems where the inducer motor would kick on but after 1-3 minutes turn off. The LED on the board had two flashes which indicated "inducer motor on with pressure switch open." After hours of troubleshooting which included removing tubes from pressure switch and making sure obstructing. Checking for obstruction in exhaust pipe and running shop vac through each end, cleaning out condense tubes I still had two flashes. I finally removed tube from pressure switch to combustion box and ran heat cycle and it worked.however during that cycle warm air would only blow for 15-30 seconds and then blow just cool air. Once I reconnected the tube it would go back to the two flashes. I noticed some grime and residue below the inducer motor so I removed it and noticed the plastic gasket was cracked and brittle. I removed the gasket it fell apart once I touched it, cleaned up the area and reseated with high temp silicone sealent . I no longer get the two flashes and the furnace runs through it's normal cycle BUT I still have a problem where the furnace is only pushing out cool air. The flames do not stay on long at all. I did remove the flame sensor and cleaned it but I fear I may have cleaned it too much by way of over use of sand paper.
I replaced the flame sensor and the limit switch and still having same issue.
AUTHOR
Dan Reed
22 months ago
Ray, Sounds likely the pressure switch is either bad or doing it's job because the chimney is blocked or the fan is running too slow to satisfy the pressure switch. Pressure switch is where I would start my assessment.
AUTHOR
Dan Reed
22 months ago
Vick, It sounds to me like if this problem occurred at the same time as your repairs, you've gotten something in the wiring incorrect. I would advise you revisit this to be sure. Is it possible you have the transformer wired to the board in the wrong spot thus applying constant power to the circuit?
ray
22 months ago
furn inducer comes on then indoor fan comes on and blinks 5
Vick
22 months ago
I installed a new gas valve and transformer. I have no thermostat control. Furnace running constantly
AUTHOR
Dan Reed
22 months ago
Colleen - If that is the case you definitely have an airflow issue but what that issue is, I can't be certain from here. It sounds as if there is a lack of return air but what's tripping as a result and causing the unit not to light...??? I suppose if it was REALLY sucking hard for air it could be pulling the gas away from the burner bar, downward, and to the point where it's not getting to the igniter but I would wonder why now and not before. Have you done any work to the system lately?
AUTHOR
Dan Reed
22 months ago
Ryu - If it clicks and nothing else happens, it may be the inducer motor. That is the first thing to come on and if there is a problem there, it won't do anything else.
Colleen
22 months ago
Furnace was blowing cold air, had a tech. come out, he checked everything and all was ok, but if the door was off the furnace would light, put door back on and it would not ignite. Tech thought it may be an air flow issue or door sensor fault, but really couldn't figure it out???
ryu
22 months ago
Thank you for your time Dan, you are so nice to try to help everyone saving money here. When I turn it on. The control board red light is on and have one clicking sound on the board after 1sec. The light is still solid on, but nothing going to work. Is it the board problem or some cable problem ? Looking forward for your reply. ^_^

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Troubleshooting Common Furnace Problems With an HVAC Expert

Figuring Out Furnace Problems

Furnace Troubleshooting Checklist

1. Check the Thermostat

Thermostat Problem Checklist

How to Test a Thermostat

Mercury-Controlled Thermostats Gotta Go

Replacing Your Thermostat

2. Check for Power to the Furnace

Did the Furnace Blow a Fuse?

3. Make Sure the Gas Is On

Did You Check the Filter? (I Had to Ask.)

Though it doesn't cause as many problems for heat as it does A/C, did you check the furnace filter?

4. Check the Flashing Red Light

5. Check for Blockages

Furnace Pressure Switch Problems

6. Check Furnace Ignition

A Furnace Flame That Keeps Going Out

Calling a HVAC Professional

7. Know When to Call an HVAC Professional

Questions & Answers

Related

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Water Heater Repair: Troubleshoot and Replace Thermostats and Elements

HVAC Gadgets & Universal Parts: Tools for Furnace Repair Pros

Furnace Won't Light? How to Replace a Furnace Ignitor

Install Your Own Line Voltage Baseboard Heater Thermostat

Popular

How to Diagnose Furnace Problems & Why the Red Light Is Blinking

Everything You Need to Know About a Furnace Pressure Switch

How to Quiet a Noisy Furnace

Comments

Home Appliances

About Us

This website uses cookies

Troubleshooting Common Furnace Problems With an HVAC Expert

Figuring Out Furnace Problems

Furnace Troubleshooting Checklist

1. Check the Thermostat

Thermostat Problem Checklist

How to Test a Thermostat

Mercury-Controlled Thermostats Gotta Go

Replacing Your Thermostat

2. Check for Power to the Furnace

Did the Furnace Blow a Fuse?

3. Make Sure the Gas Is On

Did You Check the Filter? (I Had to Ask.)

Though it doesn't cause as many problems for heat as it does A/C, did you check the furnace filter?

4. Check the Flashing Red Light

5. Check for Blockages

Furnace Pressure Switch Problems

6. Check Furnace Ignition

A Furnace Flame That Keeps Going Out

Calling a HVAC Professional

7. Know When to Call an HVAC Professional

Questions & Answers

Related

Popular

Comments

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This website uses cookies