Troubleshooting Common Furnace Problems With an HVAC Expert

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

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.

1. Check the thermostat settings.
2. Check for power via the breaker, switch, indicator light, and fuse.
3. Check that the inline gas valve is open.
4. Check filter.
5. Check ignitor for ignition.
6. Check flame sensor for ignition confirmation.
7. Check for flue/chimney blockage: pressure switch.
8. 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

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.

1. Locate the circuit board where the thermostat wires connect to the furnace.
2. 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.
3. Connect your jumper wire between the "R" and "W" terminals.
4. 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).

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.)

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

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.

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.

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.

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...

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.)

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.

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.

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...

Question: Why does my heater blow cold air?

Answer: Something has failed and the blower is kicking on as a default, most likely.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

© 2014 Dan Reed

Comments

Phyllis Celli on July 09, 2020:

You have an excellent questions & Answers page!!! I live in an apartment complex and my heating system is right in the middle of our living space, etc, This heater/air-conditioner is so very very loud & disturbing - Other than reporting to Maintenance, it was stated that these P&N WeatherKing Heaters are Loud. I BEG you to please help/aide me-it is extremely bothersome - other units like mine are the same.....Kindly comment and make suggestion!!!! THANK. YOU

Charisse Anderson-Wolfe on June 18, 2020:

I have no power to my furnace or thermostat? My ac wasn’t working and my husband put in basically all new parts, now the thermostat won’t come on and nothing happens. He put a new control module on the heat, because the tech said it was going out last winter. Don’t know what to do? No power to our Coleman Forced Air Furnace. Please help!!!

Scott on April 12, 2020:

I have a Honeywell pilot valve question, since it started recently having trouble getting the pilot to light and it continually got worse to where it will not light now. I have checked that the S8610 spark module is sending the proper 24v to the pilot valve, but I'm not sure it is opening the gas flow. I have an old manometer and hooked it to the outlet side port and cycled the furnace again. The igniter starts but I got no pressure on the outlet. Is this a proper test or does the main valve have to open before the outlet side can be tested. Thanks.

Kay Baker. M on March 01, 2020:

My furnace is clicking on but the auto fan does. Not come on

Mike L on January 17, 2020:

"My Goodman gas furnace blows a fuse as soon as the burner lights. What would cause this?"

Has there been any feedback from the original question, my goodman is doing the same thing, and if not where would be a likely place to start 1st, any suggestions would be very helpful.

Dominic on October 15, 2019:

Hi i have a goodman its flashing red light. 4 flashes what does it mean

TIM OCONNELL on October 02, 2019:

If have power coming out of the on off switch on my furnace and the 5 amp fuse is still good, why wont the green indicator light come on? Furnace acts like its dead.

Robert Estrada on August 10, 2019:

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 on June 03, 2019:

I have a new furnace and the inducer will kick on and off while in cool mode

Chuck on May 18, 2019:

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 on May 16, 2019:

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 on May 07, 2019:

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 on May 03, 2019:

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 on February 24, 2019:

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 on January 29, 2019:

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 on January 07, 2019:

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 on December 22, 2018:

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?

Dan Reed (author) on December 04, 2018:

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 on December 03, 2018:

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.

Dan Reed (author) on November 26, 2018:

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 on November 25, 2018:

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.

Dan Reed (author) on October 24, 2018:

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 on October 23, 2018:

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.

Dan Reed (author) on October 23, 2018:

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 on October 22, 2018:

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!

Dan Reed (author) on September 28, 2018:

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.

Dan Reed (author) on September 28, 2018:

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 on September 27, 2018:

My furnace blower is running but not blowing out heat, my thermostat is off, what do I do?

Tracy G on September 27, 2018:

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?

Dan Reed (author) on September 23, 2018:

Brad W - Unfortunately there isn't enough information for me to help. Maybe the stat is bad? Maybe a limit? Sorry.

Brad W on September 22, 2018:

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

Dan Reed (author) on September 11, 2018:

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 on September 10, 2018:

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??

Dan Reed (author) on August 28, 2018:

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 on August 27, 2018:

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.

Dan Reed (author) on August 27, 2018:

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.

Dan Reed (author) on August 27, 2018:

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 on August 26, 2018:

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 on August 26, 2018:

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.

Dan Reed (author) on August 26, 2018:

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 on August 26, 2018:

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.

Dan Reed (author) on August 15, 2018:

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 on August 14, 2018:

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?

Dan Reed (author) on May 31, 2018:

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 on May 31, 2018:

My circuit board keeps getting fried and the breaker not sure why any thoughts

Dan Reed (author) on May 16, 2018:

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 on May 15, 2018:

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?

Dan Reed (author) on May 08, 2018:

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 on May 07, 2018:

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?

Dan Reed (author) on May 02, 2018:

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 on May 02, 2018:

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.

Dan Reed (author) on May 02, 2018:

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 on May 01, 2018:

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

Dan Reed (author) on May 01, 2018:

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 on April 30, 2018:

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?

Dan Reed (author) on April 30, 2018:

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 on April 29, 2018:

My furance flames come on-but blower wont run-unless I keep turning it on by thermostat switch??

Dan Reed (author) on April 24, 2018:

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 on April 24, 2018:

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?

Dan Reed (author) on April 10, 2018:

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.

Dan Reed (author) on March 19, 2018:

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 on March 18, 2018:

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 on March 16, 2018:

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

Dan Reed (author) on February 26, 2018:

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 on February 26, 2018:

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

Dan Reed (author) on February 23, 2018:

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 on February 22, 2018:

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,

Dan Reed (author) on January 27, 2018:

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 on January 26, 2018:

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

Dan Reed (author) on January 21, 2018:

It's possible and if so then make sure you get stainless steel burners since you have LP...they'll last longer.

Darlene Statz on January 20, 2018:

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?

Dan Reed (author) on January 17, 2018:

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.

Dan Reed (author) on January 17, 2018:

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 on January 17, 2018:

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 on January 16, 2018:

I have an amana furnace and its flashing a red light 6 times

Sanjay on January 06, 2018:

SO SO Helpful!! THANK YOU

Ryan on January 06, 2018:

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.

Dan Reed (author) on January 02, 2018:

Cj - I'm sorry but there just isn't enough here for me to offer any real sound advice.

Dan Reed (author) on January 02, 2018:

TD - I really can't say from here. You'd have to run tests to see if that's the cause.

TD on January 02, 2018:

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 on January 01, 2018:

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.

Dan Reed (author) on December 30, 2017:

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.

Dan Reed (author) on December 30, 2017:

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.

Dan Reed (author) on December 30, 2017:

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 on December 29, 2017:

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 on December 27, 2017:

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 on December 26, 2017:

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

Dan Reed (author) on December 26, 2017:

van - Well the ignitor is good but beyond that I cannot offer any advice based on this information. Sorry.

van on December 25, 2017:

my older carrier gas furnace ignitor glow but the won't turn on

Pamelarenae on December 20, 2017:

Thank you so much for your help. We will contact landlord again and send letter addressing also.. Merry Christmas

Dan Reed (author) on December 20, 2017:

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 on December 20, 2017:

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 on December 19, 2017:

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.

Dan Reed (author) on December 14, 2017:

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 on December 13, 2017:

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

Dan Reed (author) on December 08, 2017:

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.

Dan Reed (author) on December 08, 2017:

Ryu8 - Glad to hear!

Ryu8 on December 07, 2017:

Problem fixed. Thank you so much.

Dave on December 05, 2017:

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.

Dan Reed (author) on November 29, 2017:

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.