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Problem: Furnace comes on, doesn’t run long then shuts off. Will work if thermostat is turned way up.

Problem: Furnace comes on, doesn’t run long then shuts off.  The furnace will work if thermostat is turned way up.

Answer: This sounds like a thermostat problem or an over-heating problem. I would suggest taping the furnace door safety switch closed so it allows the furnace to stay on so you can test the controls. Test with a volt meter set to volts AC and see if you are getting 24 to 28 volts between the W and C (com) terminals on the control board from the thermostat when the furnace shuts off. If you see that 24 volts goes to zero between W and C when the furnace shuts off then the thermostat is cutting the furnace off and you have a thermostat problem. If you continue to get 24 to 28 volts between W & C then you have another control shutting the furnace off. This could be a safety control like a limit, rollout switch or pressure switch cutting the furnace off. Test the limit with you volt meter. If the limit is going to zero (open) then you would have a furnace over-heating problem which could be caused by a slow blower motor, (please check the capacitor) could be a stopped up filter, AC evaporator coil or the gas valve maybe outputting too much gas into the furnace. Please click here if you are interested in seeing the furnace limit and rollout switches we sell. Below we have a YouTube video that was made by Grayfurnaceman which shows how to troubleshoot a furnace limit. I hope this video will help you out. Thanks to Grayfurnace man! Thanks so much for your interest in our website! If you have any questions please feel free to email us at: arnoldservice@gmail.com or respond in the comments section near the bottom of this page. We hope you have a blessed day! Steve & Barbara Arnold

8 thoughts on “Problem: Furnace comes on, doesn’t run long then shuts off. Will work if thermostat is turned way up.

  1. I have a carrier gas furnace it will run for about 90 seconds and then shut it self off and shows code 34. It will then reignite after about 15 seconds and run another 90 seconds and continue that pattern until it reaches the thermostat temperature. If I take the front panel of the furnace off it has no problems at all and win until the thermostats desired temperature is reached. Have you ever come across this problem, or have any ideas of what could cause this?

    1. Hi Roger! Code 34 from what I have is a flame sensing problem. I would like to suggest that you clean your flame sensor and make sure all wire connections are good and tight. There is a YouTube video about Code 34 on the following page: https://www.youtube.com/watch?reload=9&v=pzQHU6pjRv0 We have a flame sensor pages here: https://arnoldservice.com/problem-i-purchased-a-flame-sensor-from-you-for-my-goodman-furnace-the-new-flame-sensor-did-not-fix-the-problem-my-furnace-still-lights-for-7-to-10-seconds-and-then-the-gas-goes-out-what-could-be/ and https://arnoldservice.com/problem-your-furnace-main-burner-ignites-the-main-burner-stays-on-for-8-to-10-seconds-then-shuts-right-back-off/ I would also recommend making sure that your limit is staying closed when your furnace burners shut off. If your furnace is shutting off on limit then this could be caused by a dirty filter, dirty blower wheel, stopped up evaporator coil, or slow blower motor (might need a new blower motor capacitor). We have a post about this on the following page: https://arnoldservice.com/problem-why-does-my-furnace-go-off-on-limit-so-much-why-do-i-have-to-replace-my-furnace-limit-switch-so-often/ I hope you can easily find and fix the problem. Steve

      1. I appreciate the quick reply Steve, yeah I’ve actually had a HVAC tech out and he spent 3 hours looking over everything and he couldn’t figure it out either. The system is only about 16 months old and everything looks good. The puzzling part is it runs absolutely fine with closet door open and the panel off the system. I thought maybe something could be getting overheated without the added ventilation of the panel off and door open.

      2. Hi Roger! Make sure that the pressure switch is staying closed when the furnace cuts off. Maybe they did not size the vent correctly and you might have too long of vent piping or too many turns. I would suggest looking at the manufacturer’s recommendations for the length of the vent pipe and turns allowable. If your furnace is over-heating the limit will open and your furnace will shut down. Check with a voltmeter to make sure that the limit is not opening up when your furnace shuts down unexpectantly. I hope you can find and fix the problem soon. Steve

  2. I have a goodman furnace flame goes out just before the fan kicks in fan stays running true the rest of the attempt it will light but go out

    1. Hi Paul! I would suggest that you check your safety controls like the pressure switch, limit and rollouts to make sure they are staying closed when the gas goes out. You will need a voltmeter to do the testing. I would suggest testing to make sure you are getting a constant 24 to 28 volts AC between the C and W terminals on your control board. If the voltage is dropping out during a heating cycle then you probably have a thermostat problem. If your thermostat has batteries please make sure the batteries are good. I would like to suggest that you check all wire connections and ground wires to make sure they are all good and tight. We have lots of furnace troubleshooting advice on the following page: https://arnoldservice.com/gas-electric-furnace-troubleshooting-simplified-2/ I would suggest going through the troubleshooting flow chart to see what is wrong: https://arnoldservice.com/wp-content/uploads/2014/12/furnace-troubleshooting-flow-chart.pdf We also have lots of troubleshooting questions and answers on the following page: https://arnoldservice.com/furnace-troubleshooting-questions-answers-index/ If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem.

  3. I have a lennox furnace modelnumber G12Q3-137 it keeps coming on and shutting off every 5 seconds i changed the thermostat, thermocoupling and the transformer thats what i was told it was but it still is doing it what else could it be thank you

    1. I am not familiar with Lennox furnaces but if you have a spark ignition furnace it could be the control board that is the problem. I have had seen this problem with other spark ignition furnaces. Sorry, I do not have a Lennox parts program where I can look up parts. It sounds like you are definitely having a flame sensing problem since the furnace is cutting off within 5 seconds. I would suggest making sure all wire connections are good and tight especially the ground wires. Make sure that the burners are good and clean and burning a blue flame. If your furnace has a pressure switch make sure the pressure switch is staying closed the whole time the draft inducer is on. If your control board has a flash code try and read the code to see what it is showing is wrong with the furnace. We have lots of furnace troubleshooting advice on the following page: https://arnoldservice.com/gas-electric-furnace-troubleshooting-simplified-2/ I would suggest going through the troubleshooting flow chart to see what is wrong: https://arnoldservice.com/wp-content/uploads/2014/12/furnace-troubleshooting-flow-chart.pdf We also have lots of troubleshooting questions and answers on the following page: https://arnoldservice.com/furnace-troubleshooting-questions-answers-index/ If you have any specific questions please let me know. Thanks! I hope you can easily find and fix the problem.

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