Problem: Furnace comes on, doesn’t run long then shuts off. 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. 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: firstname.lastname@example.org or respond in the comments section near the bottom of this page. We hope you have a blessed day! Steve & Barbara Arnold