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Furnace Troubleshooting flow chart

This gas furnace troubleshooting flowchart should help you troubleshoot and see what is wrong with your furnace. Just follow the steps. You will need a volt meter for testing. The picture below is pretty small, especially if you do not have any zooming capabilities with your computer. We have a link for a downloadable PDF file if you want to print the chart out so you can read it and take it with you while troubleshooting. Please click on the following link if you would like the downloadable PDF flow chart.: furnace troubleshooting flow chart If you have any questions please email us anytime. Our email address is:  We would love to help you out and have your business!


42 thoughts on “Furnace Troubleshooting flow chart

  1. My gas furnace runs great until December. Then it tries to start up and shuts off, tries again 2 more times and shuts off each time. Then the furnace shuts down. I go down with an old tooth brush and clean the heat sensor with the switch off. Turn the switch back on and it fires. It can do this periodically or 3 times in a weekend. Sometimes I just turn the switch off, wait a few, turn it back on and it will fire. My worry is that one day this will happen after I’ve left for work. The heat would then be off all day until I got home. Please help.

    1. Hi Gail! Sounds like you are having furnace flame sensing problems. We have a flame sensor troubleshooting post of the following page: The Youtube video near the bottom of the page is good for troubleshooting flame sensor problems. If replacing the flame sensor, and checking all wire connections to make sure they are good and tight does not work then you might need a new control board to fix the problem. We have lots of furnace troubleshooting advice on the following page: I would suggest going through the troubleshooting flow chart to see what is wrong: We also have lots of troubleshooting questions and answers on the following page: If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem. Steve

  2. What is the best way to learn about electrical on a furnace and the air conditioner

    1. The best way if you wanted to go into business would be to go to a technical school that is NATE certified. If you want to learn as a DIY person then I would recommend following the Youtube videos on the AC Service tech channel. Here is a link to that Youtube channel: He has some great Youtube videos on air conditioning and furnace repair. I hope you have a great day! Steve

      1. Hello, have baseboard gas heat broiler, the pump went and was changed, ever since the heat will not kick on unless I bump thermostat up to 75, and when it does kick on it works really well, and need to shut it off after about half hour, becomes way too hot in house. We also changed thermostat, no luck, any suggestions, never had any issues at all until we had to change pump.

      2. Hi! I am sorry, but I really do not know much about boiler heating. The only thing that I can think of that might be causing this problem with the thermostat would be maybe a loose low voltage wire connection or the new boiler pump might be drawing too many amps for the thermostat to handle. In other words, the new pump could be throwing the heat anticipation/anticipator off on the thermostat. Most of the time a new digital thermostat will fix this problem. If you are interested here is a link to a good Youtube video on setting thermostat anticipation: Sorry that I can not be of more help. I hope you can find and fix the problem soon! Steve

  3. Steve, I have a Nordyne FG6RC-080C-12B (happy to send you a pic). Just replaced (within last week), pressure switch, limit switch, control board, flame sensor, flame igniter and a new Honeywell Thermostat.

    When i call for heat the furnace kicks on; steady red LED (power), then yellow LED (flame), burner box heats up, runs for several minutes, shuts down, 2 red flashes (pressure switch), then one red flash (limit circuit), then furnace cycles and operates fine again – does this until desired heat is reached.

    I notice gurgling (suction of water) from a drain tube that is connected to the bottom of the header box (black box behind blower motor) – which i think should be draining into the trap – not sucking.

    Any thoughts?

    1. Hi David! I would suggest that you check with a volt meter to see if the limit is opening up or if the pressure switch is opening up as the flash code suggests. I would suggest that you disconnect the pressure switch sensing tube and use a wet vac on the furnace’s condensate drain line to make sure that the furnace’s drain is open. If you can pour some water down through the furnace’s drain then this is a good test to see if the drain is open. We have a pressure switch troubleshooting page that I hope will help you out: and we have a post about troubleshooting a limit switch on the following page: We have lots of furnace troubleshooting advice on the following page: We also have lots of troubleshooting questions and answers on the following page: If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem. Steve

  4. Hi Arnold , I got York furnace in the attic and goes out on auxiliary limit switch after it ran for couple hours . I had to Manuelly reset every time . This switch is located in blower compartment . Please help

    1. Hi Pete! More than likely if you have a down-flow furnace either you have a blower motor that is going off before getting all the heat out of the heat exchanger or you have some kind of restriction in the blower motor or heat exchanger that will not allow the heat to be expelled out by the blower. The blower motor might not be running long enough on a downflow furnace to get the heat out of the heat exchanger, thus the heat comes up and causes the limit to go off. If you have a fan control where you can adjust it, then try to adjust the control so the fan will run longer to get the heat out of the heat exchanger. I would also suggest that you make sure the blower motor capacitor is in good condition. I hope you can easily find and fix the problem.

  5. I enjoyed reading this. Thank you.
    I have an York Stellar that was put in in 1989. This fall it started acting up. It runs fine for a few hours then fails to fire on heat call. Just replaced the flame sensor.( Cheap ) with no change in its bad function. When it does work it lights-fires and completes a cycle. The code seemed to be show red flash when it fails. I checked the incoming and outgoing flue and did remove a VERY old filter that was sucked into the furnace at some point but was stuck before hindering the blower squilcage.. It is frustrating but I did find YORK parts are easily available or in stock.. Just no clue what needs replacing. FYI The burners are a beautiful blue with no yellow when lit. HELP… small town HVAC dude is worthless.( Third visit from him and no help at all.)

    1. Hi Davis! I would suggest that you try and read the flash code on the control board to see what the control is showing that the problem is. The flash code interpretation should be located on the furnace blower door or somewhere on the furnace. After you find out what the board is saying the problem is then you can concentrate on correcting the problem with new parts or cleaning parts. I would suggest making sure all wire connections are tight for a start. We have lots of furnace troubleshooting advice on the following page: I would suggest going through the troubleshooting flow chart to see what is wrong: We also have lots of troubleshooting questions and answers on the following page: If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem. Steve

  6. I have an older carrier gas furnace. The inducer starts and the pilot comes on, but no flame. I ran a115v from terminal R to terminal 5 and the flame started and the furnace ran perfectly fine. Does this mean that the gas valve is good, and there is another problem?

    1. Hi Tammy! Many of the gas valves have two solenoids inside the gas valve. The PV (pilot valve solenoid) and the MV (Main gas valve solenoid) Yes, since the PV (pilot valve) on the gas valve is opening, your pilot is lighting and the MV (Main Valve) gas valve is coming on with you hot wiring it, then this would mean that the gas valve is good. Sounds like you might have a problem with the control board or other electrical problem that is not allowing 24 volts to get to the main gas valve. I would suggest making sure all safety controls are closed like the pressure switch, limit and rollout switches. I hope you can easily find and fix the problem. Steve

  7. This is exactly what I needed, thank you.

  8. Hi, my natural gas furnace is about 3 years old. I have now noticed that the high and lower speeds no longer work. It just blows on high, no matter what speed you put it on. It also blows cool air for about a min, than shuts off, then blows warm air. Please advise

    1. I am sorry, but I do not know for sure what the problem with your furnace could be because I do not know how the installers hooked up the thermostat on your furnace. Some two speed furnaces use a 2-stage thermostat to switch between high and low speeds so this could be a thermostat problem. Some installers use a regular singe stage thermostat and allow the control board to control the time the furnace runs in low and high speeds. So you could have a control board problem. I have our Bryant 90I home furnace’s control board set to have the speeds, low-fire and high-fire determined by the control board. If our furnace runs longer than 10 minutes on low speed the high speed automatically kicks in. The control board determines the number and length of the heating cycles and determines whether the furnace needs to be in low or high heat. If the thermostat or another control is short-cycling (not allowing long run times) then your furnace’s control board may have determined it needs to be in high heat because of the short cycles. The thermostat or another control could be causing this problem. I would suggest starting with the easy solutions first by checking all wire connections and plug-in connections to make sure they are good and tight. I would suggest testing your limit to make sure it is staying closed and not short-cycling the furnace. If everything checks out OK then you might need to call a service tech because these new 2-speed furnaces can be complex. I hope you can easily find and fix the problem. Steve

      1. Thankyou Steve !

      2. It worked good untill I moved the thermostat. I had to add an extension on to the wires and like a dumby I didn’t turn the power off, so one of the thermostat wires touched another bare wire. So now I believe I fried the mother board. It is a Miller and was professionally installed

      3. Hi! The only way you could have hurt the control board would be if there is a common wire on the thermostat (five or more wires going to the thermostat). If you touched one of the other wires with the common wire it could burn up a component or cause the fuse to blow on the control board. If this happened I hope your control board has a fuse to protect the low voltage components. I hope you can get if fixed. Merry Christmas! Steve

  9. I have a train XL 80 furnace that is 23 years old, everything works fine except the power vent pipe seems excessively hot. (200° at 12 inches above the furnace). What should the temperature range be on the power vent pipe during heating cycle at the top of the furnace.? Too hot could be a cracked burner, but the blower and the Hi/low vacuum switches work perfectly. If there was a cracked burner would it not be getting enough vacuum to operate properly? The power vent pipe is 14 feet straight out with one elbow and is not blocked.

    1. Hi Doug! I would think that on an 80% furnace the vent pipe should be pretty hot. Hot enough to burn you. I do not know what the normal temperature of the vent pipe should be. The furnace has safety rollout switches, limit and pressure switch that should protect you from a dangerous situation. If the heat exchanger is leaking it should trip one of the roll-out switches and yes affect the pressure switch where it would not work properly. Maybe you have the gas input set too high. If you are getting real hot air out of your registers then you might think about adjusting the gas pressure down a bit or placing your furnace blower motor on a higher speed. This can be dangerous in doing this because if adjusted too low it can cause delayed ignition which would cause a slight explosion. I have seen furnace doors blown off from delayed ignition. You might want to get a tech to check it out if you are concerned. Sorry that I can not be much help. Steve

  10. Hi

    I have a Lennox High Eff. furnace that is approx 22 years old. Problem is that the heat shuts off and blows cold air. I have had a tech try to fix for the last two weeks and each time they try a new part and it works for different lengths of time, then the problem comes back. So far they have replace the t-start, control board an air diaphram, and now the gas valve as recent as yesterday. Upon installing it worked when they left yesterday, but at some point in the night it stopped heating again and blew cold air all night. Not sure what the problem might be. Also, I am so far into this with cost to repair and it is still not working. Any information would be a help. My e-mail is

    Thanks Larry

    1. Hi Mr. Parker!
      Most of the time when a furnace shuts off and blows cold air it is because the furnace has over-heated. The control board thinks the furnace is too hot so it turns the blower on constantly to cool the furnace down. I would like to suggest that the next time that this happens that you check all the roll-out switches and limit to make sure they are not open. We have a post about this problem on the following page: Other problems that can cause a limit to open up would be poor air flow like a dirty filter, a stopped up evaporator coil a slow or dirty blower wheel (a weak blower motor capacitor) You might need a new blower motor capacitor if the motor is slow. I hope you can easily find and fix the problem. Steve

  11. Hi Steve, I have a goodman GMVC80704BXAA gas furnace. During start up the burner igniter will not power up. If I short out both of the pressure switches after the inducer motor starts up, then the igniter will power up and the burner will fire up. It runs normally until the next power up cycle, but again it will not fire, unless I short out the switches. If I apply vacuum to either switch, I hear a clicking sounds, but I get no continuity. Does this indicate both pressure switches are bad?

    1. Hi Bob! This sounds like you either have a pressure switch problem or a induced draft problem. We discuss this on the following page: Most of the time on a two stage furnace like you have the 1st stage pressure switch closes first, but sometimes the draft inducer builds up to high speed (high heat), both pressure switches close, until the flame ignites and then kicks back to low stage. I am sorry, but I do not know the sequence of operation on your furnace as to whether both pressures switches close at first or just one. Our Goodman parts program shows that your furnace uses pressure switch part number: 0130F00049. I am sorry we do not stock this pressure switch. Please follow the advice in the post I sent above. If none of these are the problem then I would suggest a new pressure switch. If you want me to send you a complete parts list for your furnace please email me at and I will be glad to try and help. I hope you can easily find and fix the problem. Steve

      1. Hello
        My furnace does the opposite, it ignites no prblm, aftr it ignites the flame runs 3 or 4 secs an shuts off an the cycle strts over again, it will do this 4ever. Anothr prblm is when it does stay lit it runs constant so then overheats an blows cold air… An thats when the prblm strts that i told u 1st… I was thinkin of usin the trbleshootin flowchart…

  12. Burner comes on, blower comes on. Burner continues fine while the blower comes on and off repeatedly. The cycle continues until the thermostat is satisfied.
    High limit switch?

    Thanks in advance!

    1. Hi Steve! This sounds like this could either be a blower motor problem or a control board problem. I would like to suggest that you test with a volt meter set to “Volts AC” the connections that come out of the control board that go to the blower motor. You would test on terminals marked “Heat” and one of the white neutral connections. You will probably need to tape the furnace blower door safety switch closed so you can do testing with the power on. Set the thermostat so the heat is on and notice if the power goes off and on (120 volts to 0 volts) to the blower motor or if the blower motor gets constant 120 volts and still goes off and on. If the voltage drops out of the control board then you probably have a control board problem. If the voltage stays steady and the motor still goes off and on then you probably have a blower motor problem. I would suggest making sure the blower motor capacitor is in good shape. I do not believe it would be a limit switch because if a limit is open the blower usually runs all the time. If you have an old fan limit control like the Honeywell we sell on the following page: then yes, it might be a fan limit control that is giving you this problem. Most of the furnaces now have a control board that controls the fan. I hope you can easily find and fix the problem. Steve

  13. My furnace will call for heat, turn on the inducer motor and then will not fire the burners. I have tested the voltage from the gas control valve to the pressure switch and it reads 23 volts. Out of the pressure switch it reads around 15. I have tried to jumper the wires off the pressure switch and had the same results. I am getting 27.8 volts into the gas control valve, but for whatever reason, I don’t ever get 24 volts to or from the pressure switch. I even replaced the pressure switch and got the same results. Any help is greatly appreciated.

    1. Hi Eric! Sorry to hear that you are having trouble with your furnace. Since you are not getting 24 volts out of the pressure switch I would suggest that you make sure that your low voltage transformer is grounded well to the frame of the furnace. Sometime a loose grounding screw can cause low voltage. Is your ignitor glowing because on most furnaces if the pressure switch does not close the ignitor will not glow. Another thing to look at would be any connections from the pressure switch to the control board. Make sure all connections are tight and not burnt. Sometimes a molex connector will be burnt or loose and cause the furnace not to work right. I would suggest testing with a volt meter set to “volts AC” between the R (red) and C (com) connections on your control board. You should be getting 24 to 28 volts AC during furnace start up. If you aren’t then you might need a new transformer or have a loose ground wire. I hope you can easily find and fix the problem. Steve

  14. when the flame kicks on to heat up before the blower comes on, why does it sound like there is air in it (only way I know how to describe it) then it goes back to normal.

    1. Hi Judy! The only thing that I can think of that might cause this would be, maybe the burners need to be cleaned or there is too much gas pressure going to your burners. You might have a tech check to make sure the gas pressure isn’t too high. Sorry, but that is all that I can think of. Hope you get it fixed soon and inexpensively. Steve

  15. Ignitor is new, voltage to ignitor is 120v, flame sensor id clean. Blower runs for approx 10 seconds, no glow at ignitor, then shuts down. Fuse ok at the control board. Thermostat has 24 volts in and out. What else should I check? THANK YOU.

    1. Hi Nick! You say the voltage to the ignitor is 120 volts and the ignitor is not glowing? Please check the voltage coming out of the control board with the ignitor plugged in if possible. Sometimes the voltage going to the ignitor will show 110 volts without the ignitor plugged in and when you apply the load of the ignitor the voltage will drop to zero showing that you have a bad relay on the control board. If you have 120 volts coming out of the control board to the ignitor with the ignitor plugged in, then you must have a bad ignitor because it should be glowing with the voltage applied. If the ignitor does not glow at all then I would suggest checking to make sure all safety controls are closed like the limit switch, roll out switches and pressure switch. The first things that a control board checks for in the ignition sequence is to check and make sure the draft inducer is running and the pressure switch is closed. If the limit and rollouts are closed the control board sends 110-125 volts to the ignitor. If any of the safety control are open then the control board will shut down the furnace. I hope you can easily find and fix the problem. Steve

  16. Installed new blower with add on capictor,old motor did not have an external furnace warms up shuts off after a few min and blower comes cycles like this till thermostat is satisfied.

    1. Hi Joe! Most of the time a blower motor with a capacitor is better and more efficient. Sounds like you might need to adjust your fan limit control if you have this type of fan limit. If you have an older furnace most of them have adjustable fan controls like the Honeywell controls we have on the following page: The newer furnaces control the timed off/ON of the blower motor through the control board. I hope you have not, but you might have the blower motor wired incorrectly. Please make sure you follow the wiring diagram on the motor label. I hope you can easily get this problem corrected. Steve

  17. Furnace runs for an awful long time after flame goes out to cool the heat exchanger is there a reason why

    1. Hi Tom! High Efficiency furnaces run long times so they do not waste any heat in the heat exchanger. This is part of the high efficiency. Most furnace have dip switches on the control board that will allow you to adjust the time the furnace stays on after the gas shuts off. I would suggest that you look at the label on your furnace or the owner’s manual and see if the furnace has any dip switches that you can set to cut the run time down. I hope you have a great day! Steve

  18. My furnace runs for 5 min. Then shuts off for 2 min, then repeated the same cycle
    The flame sensor was cleaned by the furnace employee.
    Still have a problem.Do flame sensors go bad?

    1. Hi! This does not sound like a flame sensor problem because the furnace is running for 5 minutes. If the burners were cutting off in 8 to 10 seconds then I would think it was a flame sensor problem. I would suggest that you check your limit to see if the furnace is going off on limit, then cooling down, then coming back on. If your furnace is going off on limit then you might have a dirty filter, blower wheel or a stopped up evaporator coil. A slow blower motor (maybe a weak capacitor) can cause an over-heating condition. I would also check between W and C (com) to make sure the thermostat is telling the furnace to stay on when it goes off. You should get a constant 24 to 28 volts AC between the W and C terminals on your control board when the thermostat is calling for heat. Best of luck in finding and fixing the problem. Steve

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