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Situation: Nest thermostat shows that the furnace is running, but its not and no error codes are displayed on the furnace. This happens about once per day where it seems the furnace doesn't ignite properly. I finally saw the behavior first hand... The furnace actually lit on the first or second try but then went out after about 3 seconds.

If i turn down the heat, the Nest app shows that the heater isn't running, then I wait 10 seconds and turn the heat back up and the heater ignites successfully. Any ideas why it would light, then turn off and not throw an error code?

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    call a tech, it sounds like there is an issue with the burner side of your furnace, which we don't suggest DIYing because of dangers inherent in gas. Jan 25 '21 at 15:38
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    Agreed that this is likely a situation for a trained professional. At a very minimum, though, provide the make/model of the furnace - someone here may be familiar enough to walk you through troubleshooting procedures.
    – FreeMan
    Jan 25 '21 at 15:45
  • Yep, time for a pro. Sounds like a flame proving sensor is marginal or needs to be cleaned. But again, a job for a pro. Jan 25 '21 at 16:01
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    The thermostat doesn't show "that the furnace is running". It shows that it's calling for the furnace to run. It has no idea what's actually happening because it's one-way communication in this case--only proprietary thermostats would have diagnostic data available. It's very odd that the furnace shuts down without error codes. Be sure that you're interpreting it correctly.
    – isherwood
    Jan 25 '21 at 16:05
  • I don't agree with the knee-jerkers about calling a pro (just yet). There are many reasons this could be happening due to issues that are easily resolved. Please revise your question to tell us something about your furnace. You wouldn't ask your mechanic for help without showing them your car.
    – isherwood
    Jan 25 '21 at 16:07

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