Troubleshooting Common Furnace Problems With an HVAC Expert

Updated on January 19, 2017
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Dan has been in the HVAC industry for 22 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 22 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

  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.

Below, I explain each of these processes in 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

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.

Mercury-Controlled Thermostats Gotta Go

If you have the old tan, turn dial type 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.

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 they aren't loose, 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 of for testing a regular 24v (low voltage) circuit but not a high voltage one. In other words, don't use these instructions if you're not troubleshooting your furnace.

#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 (although it might be located elsewhere on the boards of certain 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. DO NOT try to substitute the fuse with one of another amp rating.

#3: Make Sure the Gas Is On

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?

See results

#4: Check the Flashing Red Light

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

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...

  • removing 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.
  • making 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.
  • 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 just like you would a water heater. If it won't light, then you probably need a new thermocoupler. These are cheap and can be bought at most hardware stores. (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.

Trying to fix a furnace yourself is great, but know when you need an HVAC professional.

Experimenting can be dangerous and cause more problems than you already have.

Questions & Answers

  • 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?

    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.

  • Why is the zone 2 light red on my Honeywell furnace? Is it not working?

    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.

  • Why does my thermostat read RECOVERY every so often?

    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".

  • My Comformaker furnace is showing a four flash code. I replaced the limit switch. Why is it not igniting?

    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.

  • 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?

    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.

  • Why does the flame stay on when a furnace is off?

    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.

  • 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?

    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.

  • The blower will run, but no burners come on?

    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.

  • 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?

    Possibly your blower motor is bad. That's what I would be checking first.

  • I jumped the pressure switch because the tube keeps filling up with moisture. What would cause this?

    Improper venting is the most common cause of excess moisture. I can't say what may be improper, but that is usually the case.

  • What does it mean when the light goes out then a flash?

    It likely means you don't have power to the unit or perhaps blew the board fuse.

  • My Goodman unit is flashing a code for a pressure switch stuck open. I have cleaned the barbed fitting where the tube attaches, replaced the tube, checked for clogged vent, and changed pressure switches. I also checked for suction by putting my finger over the end of the tube, and it seems adequate. Also tried bypassing the pressure switch. Any suggestions on what else to try?

    Sorry. Not without running tests. You've done a lot of what I would've already. You may need to get a tech out at this point.

© 2014 Dan Robbins

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      Elisabeth Maynard 11 hours 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

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      Dan Robbins 5 days ago from Ohio

      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.

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      Dan Robbins 5 days ago from Ohio

      Ryu8 - Glad to hear!

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      Ryu8 6 days ago

      Problem fixed. Thank you so much.

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      Dave 8 days 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.

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      Dan Robbins 2 weeks ago from Ohio

      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.

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      Dan Robbins 2 weeks ago from Ohio

      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?

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      ray 2 weeks ago

      furn inducer comes on then indoor fan comes on and blinks 5

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      Vick 2 weeks ago

      I installed a new gas valve and transformer. I have no thermostat control. Furnace running constantly

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      Dan Robbins 2 weeks ago from Ohio

      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?

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      Dan Robbins 2 weeks ago from Ohio

      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.

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      Colleen 2 weeks 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???

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      ryu 2 weeks 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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      Dan Robbins 2 weeks ago from Ohio

      Rachel - I am really not sure what the cause is here. The heater definitely shouldn't run without the fan on for 10 minutes though. That I'm sure of. I'd recommend calling a tech and if possible, I'd be curious what they find.

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      Dan Robbins 2 weeks ago from Ohio

      Steve - It would be hard for me to say "definitely" but it does sound logical to change the Honeywell module and is probably much easier for you to purchase at a reasonable cost and even if that's wrong and you end up buying a new control board and doing that, you'd still probably come out ahead financially if doing yourself.

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      Dan Robbins 2 weeks ago from Ohio

      Ryu - Did the light go back out on the board? It almost sounds like it's shorting out but I can say for sure. Electrical testing would need done to narrow down the possibilities. Trying to see where power is lost or getting too much/little voltage. Sorry. I know that doesn't help much.

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      Steve 2 weeks ago

      Thanks for the advice Dan. I tried to check the voltage from the board to the ignitor. My furnace has a Honeywell S8600 ignition control module. I checked the voltage from mv/pv to pv and did not get 24v. After I light the pilot with a lighter and the burners come on, I checked mv/pv to mv and did not get 24v either. Should I be replacing the control module or do u think I need to replace the board instead? If I’m not getting 24v coming into the ignition module, in that case would you say it’s definitely just the board that needs replaced?

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      rachel 2 weeks ago

      My central gas heater will not blow air during the night. It works just fine during the day. It's 40 outside and my house is freezing. The thermostat says it's 66 inside but it feels colder. I turn the desired temp up to 80 and still nothing blows out of the vents unless it's daytime. I have changed the air filter, reset the schedule in the thermostat, checked the breaker box. I cannot figure this out. I hear the heater kick on when I turn it on, but nothing blows out and 10 minutes later the heater kicks off on its own.

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      ryu 2 weeks ago

      I checked all the step is fine. I replaced my old dead control board(no light) with a new one. When i turn it on. There are only one clicking sound. Then nothing happen. What is that problem ?

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      Dan Robbins 2 weeks ago from Ohio

      Steve - Since you can light it with a lighter, the pressure switch doesn't seem to be the issue or it would not let the gas valve open. You may not be getting the proper voltage from the board to the spark igniter thus it's not lighting the gas on it's own. This would trip the flame sensor and cause the blower to come on as a safety default. This would typically happen a few times then cause the unit to go into lock out for a set amount of time before it tries again unless you reset the breaker. So, it might be the board. I would check the voltage from the board to the igniter to see.

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      Steve 3 weeks ago

      Dan,

      I have an icp furnace with an electronic pilot/ignitor assembly. The furnace comes on when call for heat, the inducer comes on but the pilot won’t light by itself to turn on the burners. If I use a lighter it will light and the furnace will run for a few minutes and the burners will shut off. Then the furnace will run continually without lighting the burners. I replaced the pilot/ignitor assembly and that still did not fix the problem. Do you think this might be a pressure switch problem? What do you think I should be checking or testing. Thanks.

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      Dan Robbins 3 weeks ago from Ohio

      First, that's the ignitor that's glowing not the flame sensor...that's on the other side of the burner assembly. Second, it will blink this way perhaps until it goes into lockout and then it will blink a lockout code. I can't say for sure but it sounds like the pressure switch. Maybe not the part itself but that whatever is going on is preventing it from getting the pressure it needs to see for the valve to open. Could be a blocked chimney, maybe the inducer motor, perhaps the part is bad...there are a few possibilities and this is just what I'd be checking if I were there.

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      Thomas 3 weeks ago

      The system is a gas unit and it won't light. The flame sensor glows but the flames never appear and then it shuts down and starts over. The led lights and fault code list says that after 4 tries it will lockout and it never did a lockout. The led light kept saying operations normal. What could this be.

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      Dan Robbins 3 weeks ago from Ohio

      Glenn - It sounds very similar to a flame sensor issue. There is another article here I've written detailing how to check it.

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      Glenn Home 3 weeks ago

      My furnace does the following: gas burner ignite for about 15 seconds, then they turn off and the fan begins. Fan runs for about 4 min. I get a short burst of warm air through the vents for about 30 seconds then it cools (room temp). It repeats the process every 5 min. Any ideas on what’s happening?

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      Dan Robbins 4 weeks ago from Ohio

      Rebecca - I'm not sure what the noise difference is in the gas but typically if it lights and then shuts down immediately it's the flame sensor which I've written another article on. Otherwise, there are a lot of things that can cause that and would need to know exactly at what point in the cycle the unit is shutting down to offer any thoughts.

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      Rebecca 4 weeks ago

      Hi Dan, thanks for having such a great blog available for people like me who don't know what's going on! I commented earlier but didn't get a response, and was just wondering if you had any thoughts?

      Basically, I have an HE furnace that has problems sometimes igniting the burners (at least, that's what the blinking red light code is telling me). I've had this problem for a while now; the furnace tries three times to light the burners, and if it can't, it then just blows cold air for an hour, after which it tries again. I've been listening to it for months and I can now tell if the furnace is going to fail by how loud the gas sound is....if it's soft, it won't work, and if it's louder it'll work. I really haven't been able to predict any kind of conditions that will cause it; I'd say 80% of the time the furnace works fine, but I can't figure out what's causing that other 20%. Any thoughts?

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      Dan Robbins 4 weeks ago from Ohio

      Gene - The leaks in windows and doors is no fault of the furnace and if they are worse than last year then it would make sense that the home isn't heating well. When is the last time you've had a clean and check?

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      Dan Robbins 4 weeks ago from Ohio

      Rich - Hard to say from here but being that new your parts are under warranty so I'd let a tech handle it. Also, the blinking light should give you an idea of what's happening.

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      Dan Robbins 4 weeks ago from Ohio

      Abdullah - I'd check to make sure the orifice the gas is coming out of is clear of obstruction that may cause low gas output....fine for a lighter but not for igniter. Then if still persists, it may very well be the igniter is going bad or not getting the right voltage so it's not getting the gas lit.

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      Abdullah Sheikh 4 weeks ago

      My gas valves are working because when I manually light the furnace when the gas is released, the valves are fully lit and do not turn off. Not turning off also indicates it’s not the flame sensor otherwise if even after manually lit, it would shut off. The problem I’m having is the igniter IS TURNING orange but furnace is not catching the flame and after a short period of time, the gas stops being sent and igniter turns orange again and tries to fire up and it just doesn’t want to catch. Maybe the igniter is faulty? Even though it is turning orange, only the bottom portion is turning orange and not the whole thing from top to bottom?

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      Gene 4 weeks ago

      It's a LP furnace and it is sucking the cold air from the doors and windows through the cracks and open area's, an the furnace feels like the heater ain't working right as I got it on 80 and the heat coming out of the floor vents is warm not hot like it was last year at this time of the year

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      Rich 4 weeks ago

      my furnace won't go over 64 & only runs for a short period of time. Its a new furnace 1 year ago. Is there a fan limit switch or something I can ck or maybe a thermostat issue?

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      Dan Robbins 4 weeks ago from Ohio

      It shouldn't. No. For what it's worth, the green wire is to control the fan. You could perhaps have a short somewhere or a place where that wire is making connection with a live wire. Likely red. It could be in the wall. Sometimes these wires can be brittle in the wall and pulling on them to replace the stat can cause the connection to short. These are guesses but are what I would be checking.

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      weathergeek27 5 weeks ago

      We have two zones. Upstairs/downstairs. Exactly the same equipment. I used the working system to double check the wiring. All is good. If the 'signal' from the old thermostat had the fan on could it be stuck in that mode?

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      Dan Robbins 5 weeks ago from Ohio

      I think you're correct that the fuse would've blown if you crossed them up. That said, I'm not sure why the blower continues to run unless there is a crossed wire somewhere in there.

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      weathergeek27 5 weeks ago

      Last night I attempted to replace my thermostat with an Ecobee 3 smart thermostat. I thought I turned off the power but it turns out I did not turn off the furnace. I am sure I touched the bare wires at some point when disconnecting and reconnecting the thermostats. I called Ecobee technical service and they helped me troubleshoot to find out that the thermostat was not getting the power it requires. After looking on the Internet I think I most likely blew the fuse on the control board because now the thermostat gets no power. I haven't checked yet. The thing is the furnace blower is running constantly on the slow setting. Would the thermostat not getting power (24 volts) becuase of a burned out fuse also allow the fan to run constantly?

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      Dan Robbins 5 weeks ago from Ohio

      sb cole - Perhaps you don't have power to the unit. The fuse is blown. The transformer perhaps or the board is bad. There are a lot of possibilities that would require troubleshooting the electrical circuit to find out where the fault is.

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      Dan Robbins 5 weeks ago from Ohio

      Jen - I can't say for sure but it sounds as if the board may be the problem. Electrical troubleshooting could answer that.

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      sb cole 5 weeks ago

      red light does not come on, what does that mean

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      Jacque 5 weeks ago

      Thanks Dan. I searched and did indeed find a loose wire in the wall behind the thermostat. Tightened it up but still have the same problem. A little more troubleshooting and I found when I checked voltage at the furnace, running 28 between R & C. When the induction motor starts, it remains unchanged and furnace ignites properly. As soon as the blower motor starts, voltage drops to 0 and flames go out. About 12 seconds later voltage climbs back to 28 and it all starts over again. No other loose wires found.

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      Jenna 5 weeks ago

      Thanks! The code says it was the pressure switch, but we replaced that part and we cleaned the flame sensor. I am thinking maybe it is a faulty ignitor.

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      Jen 5 weeks ago

      I had an initial error code that indicated the flame sensor. The furnace would sequence to the flame ignition and the after a few seconds go out. It would cycle through this 3 times and then time out. I replaced the sensor bar and now have a new code indicating sticking gas valve. I had the gas valve replaced. And still have the same code.

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      Dan Robbins 5 weeks ago from Ohio

      Jerry - It's possible the fan is running slow and thus not creating the vacuum the pressure switch wants to see so you could be right. The other possibility is you have a birds nest in the chimney...it happens a lot over the summer and no one knows until it's cold out. Otherwise, it sounds like you've been checking the items I would have too so I'd be looking at the same things and if none of that, perhaps an issue in the control board if you're sure about the other things.

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      Dan Robbins 5 weeks ago from Ohio

      Kenny - If you've removed the wires and it still runs with heat and not just the blower, then I'd be leaning towards a short in the board. The blower will run if the unit is hot and you stop the stat until it cools down then the fan would shut off.

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      Dan Robbins 5 weeks ago from Ohio

      Jacque - I don't think the transformer would do that since it's in a different part of the circuit and would have just started because you have a new stat. Sounds like a loose connection maybe that the blower is shaking the wire and if even for a second, it would cause the unit to recycle. That's just a guess but would be the first thing I would check.

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      Dan Robbins 5 weeks ago from Ohio

      James - This would definitely require someone on site with know how. Sorry I can't help.

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      Dan Robbins 5 weeks ago from Ohio

      Jenna - I and they likely can't be sure if the problem doesn't occur when they are there but if you have the LED troubleshooting light, it should tell you approximately where in the cycle the problem occurred. Might be flame sensor or pressure switch related but those are just guesses that could stem from and underlying issue. Try to check the light code before resetting next time.

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      Dan Robbins 5 weeks ago from Ohio

      Aletta - Not sure how that would be possible unless you have very poor windows, insulation, roof, etc.... These items would let your heat escape and perhaps give the impression that the furnace is the issue when it's not.

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      Dan Robbins 5 weeks ago from Ohio

      John - You may have to hold that button for up to a minute or so before letting go to make sure the thermocouple is heated. If that doesn't work perhaps you have weak gas pressure or pilot flame which you would need a tech to measure the gas and make those adjustments.

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      Jacque 6 weeks ago

      I installed a new smart thermostat. The furnace starts up normally. But as soon as the main blower starts, the thermostat loses power and reboots. After couple of minutes the furnace shuts off and restarts the lighting process, again causing the thermostat to reboot when the main blower starts. Wondering if a weak transformer is the problem where it has enough juice to run the thermostat or the blower, but not both.

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      James 6 weeks ago

      I cleaned the sensor and the furnace will run without top door on but when I put the door on the flame goes out.

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      Kenny 6 weeks ago

      My furnace constantly runs heat. I turned the thermostat off and still runs til i flip the breaker. I took the r wire off board and w off the board and still runs constantly til i trip the breaker. Any suggestions

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      Jenna 6 weeks ago

      My Kenmore furnace that is 4 years old will run for a couple of days with no problem and then all of a sudden it won't kick on. We have to flip the switch (that looks like a light switch) off wait a minute and turn it back on and it works again for a couple of days. We have had a few people look at it and they say they have no idea what it could be. Any ideas?

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      Aletta 6 weeks ago

      My furnace turns on the heat is going through the pipes my house is not heating up.

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      Jerry 6 weeks ago

      Hi;

      I have an 1985 carrier Bryant Plus 90 furnass that is not igniting do to the pressure switch. The switch has already been replaced and I have checked and checked and cleaned the inducer intake and exaust tubes, the P trap was partially obstructed and has been cleaned. Also I have removed the inducer blower assembly, cleaned up the corrosion on the blower cage and lubricated the motor. I also noticed the blower housing was not sealed firmly to the heat exchanger housing so I took care of that as well. At start up the inducer motor runs but does not engage ignition through the pressure switch and eventually turns back off. When the switch is triggered at start up, the electronic pilot/ignitor works and the furnace will run a cycle normally. My thought here is the inducer motor assembly is just not creating enough pressure anymore and needs to be replaced. It spins freely but does get rather warm while running but I'm not sure how normal that may be. Your thoughts on what to look at would be apreciated.

      Thank you

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      Jonathan 6 weeks ago

      Furnace wasn't turning on, just had to tighten the wires in my thermostat like you said in step 1. Much easier than having to call the landlord and wait for a repair man.

      Many thanks!

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      LarryG 6 weeks ago

      Have Carrier high efficiency furnace. When using switch on furnace, it goes green and furnace appears to go on for 1 second then yellow light comes on and stops the furnace from coming on. Thoughts?

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      greg 6 weeks ago

      my ignitor is not working. The cycle starts, I don't see the ignitor heating up, then gas shuts off, cycle ends.

      I have a Tappan Furnance

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      Dan Robbins 6 weeks ago from Ohio

      Dawn, It sound like a pressure switch issue or inducer motor problem. It's not really a do it yourself test of diagnosis. You're probably going to need to call someone with experience to figure out which and make the repair. Thanks for reading.

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      Dan Robbins 6 weeks ago from Ohio

      Joshua, It could be the gas valve or your gas is not on. Not a do it yourself type of repair though if you're not experienced working with gas.

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      Dan Robbins 6 weeks ago from Ohio

      Bob - It's not abnormal for these to get warm. Not burning up but warm.

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      Dawn 6 weeks ago

      Red light comes on furnace clicks , ignitor lights but gas does not start. Furnace will attempt several times then stop. Red light blinks once only. However, sometimes it starts and runs great other days it will do this several times through out the day. Any suggestions?

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      Joshua 6 weeks ago

      So my board light and everything is on and stays on, but it just seems like the ignitor and gas isn't turning on to start the heating process? What may be the problem?

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      Bob 7 weeks ago

      My furnace quit working recently. Power is getting to my furnace as the LED light is on. It's not flashing, just steady on. My thermostat appears to be functioning properly. The display is fine and the temperature adjustments work fine too. I have noticed that the low voltage transformer gets warm when the furnace switch is left on for a few minutes. Could it be warming up because it's faulty or simply because power is reaching it? Need advice and don't have much money to spend.

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      Rick 7 weeks ago

      Red light on furnace is not on, has power at the fuse and the fuse is good, any ideas on why the light would be out?

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      Dan Robbins 7 weeks ago from Ohio

      Ben - Hard to say...perhaps the light just burned out. If the unit is working then it won't likely matter until the unit isn't working then would just have to be troubleshot the old fashion way. Sorry if that's not a lot of help. Thanks for reading.

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      Dan Robbins 7 weeks ago from Ohio

      Ben - It's not something most people can do themselves since particular tools are required for testing and/or adjusting the valve. Sorry. Thanks for reading.

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      Ben 7 weeks ago

      My error light is not working and is not blinking my Furnace is coming on

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      Rebecca 8 weeks ago

      aaaand spoke too soon. Furnace seems to be having even more trouble lighting the burners. Had to restart it twice this morning before it would catch. This is very frustrating!

      But I did want to say, thank you for such an informative site.

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      Rebecca 2 months ago

      I've been having some "failed to ignite burners" issues with my HE furnace. I had an HVAC pro come inspect it, and he cleaned the flame sense rod and checked the gas pressure, all was OK. Kept having the same problem (I would hear the system trying to light the burners, unsuccessfully, and then the blower would come on and blow cold air. I'd shut the system off, turn it back on, and each time the furnace then worked perfectly).

      I called the HVAC guy back, and he said perhaps I was keeping the house too cool; I generally try to save energy by turning the thermostat down to 59 at night. He said this was causing excess condensation building up and preventing the burners from being lit. That just didn't right to me....and sure enough, it happened again today when the temperature was set to 64.

      Thoughts?

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      Dan Robbins 2 months ago from Ohio

      Jay - I'm not sure what blew. The blower comes on as a default in many cases and for various reasons when something doesn't work right. Question is, where is the "breakdown". Reset the power to the unit by killing the breaker for around 60 seconds then turn it back on and watch to see at what point in the order of things the unit seems to not do what it should. The blower may come on first while it sort of resets itself and then after a minute or two should shut down and begin it's normal cycle. To see what blew, you'd have to "hopscotch" the various components along the circuit to see where the voltage is improper and that would take a fair amount of electrical knowledge and a pretty decent voltmeter with abilities to also check for resistance and capacitance. This is what I'd have to do to find out what was going on based on this information. Sorry if it's not a lot of help. Perhaps knowing more about where in the cycle it is failing would help me more. Thanks for reading.

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      Jay 2 months ago

      Turned on furnace for the first time this season, smelt something electrical burning. Blower fan came on and stayed on. Nothing else happened during during cycling process. I used a jumper wire between the two wires on the circuit board to bypass the thermostat, still nothing other than the blower fan did anything. Do you think the circuit board is bad? Older furnace. Circuit board has no fuse or lights.

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      Dan Robbins 2 months ago from Ohio

      Eric Leach - The inducer fan (small fan hooked to the flue) or the pressure switch (disc with tube to inducer) would be my first suspects. Either one of them has failed or you may have a blockage in the chimney/flue. It's possible there could be another issue but this is what I would be looking at first. Thanks for reading.

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      Eric Leach 2 months ago

      Hi, the problem I am having is my furnace has a electric lighter and it stopped working at some point, this has happened before and I was able to clean it and that fixed the problem unfortunately this time after cleaning it the furnace wont even try to light the blower comes on right away before it even gives it a chance

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      Dan Robbins 5 months ago from Ohio

      All those could be problems however if nothing works then I'd be inclined to suspect the board is the issue. That said, I can't be sure from here but I would be thinking it. Thank you for reading.

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      I need help! 5 months ago

      Power to board but nothing comes on indoor unit nor outdoor unit. Can it be transformer or thermostat?

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      Matt 5 months ago

      So my pilot ignites, but the furnace will not ignite from the pilot unless I fan it. it then comes on no problem. Once the furnace tunrs on and heat is blowing, it will shut down 1-2 times and then comes back on no problem. I think this may be 2 problems. Any thoughts?

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      Dan Robbins 5 months ago from Ohio

      There would be no need or benefit for you to turn anything off at the furnace in the summer unless you have a standing pilot (which means the pilot light is always burning) which most modern furnaces do not have. If you do, then yes, shutting off the gas isn't a bad idea to save energy. Just remember you do it and be sure to get it lit up prior to the cold days not just so you're not cold but to make sure everything is working proper before you need it...not when. That's what leads to after hour service calls and their not so friendly costs. Thanks for reading.

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      A Grover 6 months ago

      Great article. A quick question.

      I want to switch off the furnace for the summers. I switched off the gas, and the power switch on the furnace. Now the issue is that with the power switch off, my air conditioning does not work either. Is it okay if I turn the power switch on, without turning the gas on?

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      Dan Robbins 6 months ago from Ohio

      It sounds like you may have had a short in the stat. That's a guess but you may just need to replace it. If you disconnect the stats wiring and the fan shuts off, that's a good sign that the stat is your problem. Thanks for reading.

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      David Matos 6 months ago

      The fan on my furnace runs continually and I can't control

      because the thermostat is completely blank no function but I

      replaced the batteries but no effect.

      Please what can I do?

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      abdul 9 months ago

      Thanks a lot....good information.

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      Dan Robbins 10 months ago from Ohio

      Norman Brand - I'm sorry but I can't really advise based on this info. Certainly if you have to rap on the board something is up. Loose connection? Intermittent "brown" power? Perhaps try documenting conditions and times when this happens and see if you might not be able find a commonality or at least provide the info to a tech who may be able to find one.

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      Norman Brand 10 months ago

      My thermostat will tell the furnace to come on but more and more it sits there and does nothing. If I open the service door and tap on the circuit board, it comes on and no problem. Do you think it is a problem with this board? Once it is on it works great, then a few days to weeks later, it does it again. LED is steady on. Thanks nrbrand@q.com

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      Dan Robbins 11 months ago from Ohio

      Dave in CA - Thanks for reading and offering more input. Anything to help people get their equipment going is always welcome.

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      Dave in California 11 months ago

      My furnace gave flash codes indicating that the Pressure Switch was Stuck Open. The inducer fan was turning just fine. I jumpered the two wires leading to the pressure switch, and the furnace ignited and ran fine, until it went off and then wouldn't come on again, because the "Pressure Switch was Stuck Closed", thanks to me. I removed the jumper wire.

      I disconnected the power. I removed and checked the chimney stack and found nothing. I checked the pressure switch by pulling one end of the little hose off the fan, and breathing in the free end of the hose, this made the switch open and close. I even looked up on top of the roof, the chimney was not iced over, blocked, or occupied by an animal.

      Finally, I tried hooking the little hose back up to the inducer fan, and breathing in the end that was attached to the pressure switch. I couldn't. I unbolted the inducer fan and opened it up to check inside for debris.

      Turns out the problem was that the hole inside the fan that hooks up to the hose was blocked by dust and/or corrosion. I poked it out with a paper clip and stirred it around, then tried breathing through the hose. Now I could get air in and out of the fan. I reassembled everything, including the wires to the pressure switch, and turned the power back on.

      Voila. The furnace ran perfectly.

      For your pressure switch section, I suggest a step (for older furnaces) to check to check if one can blow air in and out of the fan through the hose. If not, either the hose or the fan hole is plugged.

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      Dan Robbins 13 months ago from Ohio

      Dpreem - I can only guess that the ignitor isn't getting the correct power which would have me think the board is bad. Can't guarantee that but would be my next suspect. Thanks for reading.

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      Dpreem 13 months ago

      I have a GMPO 50-3 goodman heater. Board will cycle for about 30 secs then cut out. Steady red light on board. Cked pressure switch with suction on hose-works. Took hose off to ck and 3 blinks on board told me pressure switch open. Put hose back on back to steady blink. Changed electronic ignitor today. Flame sensor looks ok no visible problems Getting gas to unit. Took off blower inducer fan impeller looks good. Can not get unit to show a glow or get close to ignition. Is it possible blower inducer not working properly? Had a tech come out and cked most everything above said it had to be elec ignitor but said it was an easy fix, two screws and a plug. Did that today and still nothing, What am I missing?

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      Dan Robbins 2 years ago from Ohio

      Vincent - Sorry for the delay but...if you go to this link, about 50 seconds into the video, you'll see mine to the right of the inducer. I have 2 of them, you may not but those silver flat disk shaped parts are the pressure switches.

      https://dengarden.com/appliances/Flashing-Red-Ligh...

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      Vincent 2 years ago

      Hi,

      I was wondering if there was a way to find the pressure switch on my Carrier 58MCA100-16. I have the manual and for the most part I can find all the parts listed. It shows my pressure switch behind the induced blower but there is nothing there. This may sound nuts but I simply can't find it. Would you be able to direct me?

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      Dan Robbins 3 years ago from Ohio

      @ Christy Kirwan - Thank you so much for your feedback! I'm glad to be able to help in my own little way. A sort of "pay it forward" if you will. Thanks again for stopping by and commenting.

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      Christy Kirwan 3 years ago from San Francisco

      This is such a helpful guide! It's great of you to share your years of professional experience in a way that can help people save money. :)

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      mecheshier 3 years ago

      Me too :-) But I have had my fair share of gas stoves in the past. Talk about safety issues.

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      Dan Robbins 3 years ago from Ohio

      Thank you mecheshier! That's great to hear. I'm always concerned with formatting and you know, you ask yourself if it's "easy on the eyes" but who am I to say? Ha! Glad you liked it and hope you never have to use it!

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      mecheshier 3 years ago

      Great post. There are some very useful tips here. I love your format, it is easy to read and light on the eyes. I also had fun doing the quiz.

      Voted up for useful and interesting

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      Dan Robbins 3 years ago from Ohio

      @sallieannluvslife - Thank you very much for the feedback. Though I hope you're furnace doesn't breakdown on you, I hope if it does, I can be of some help.

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      sallieannluvslife 3 years ago from Eastern Shore

      Awesome Hub! Great advice to keep on hand just in case....

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