I have a pretty new high efficiency York furnace. Multi-stage NG unit.

First good cold snap last night. At 6:45 AM we get a message from our smart thermostat saying the temp has dropped 3 degrees even though we're asking for heat. The furnace is flashing with a slow red light, code: "This indicates that flame was sensed when there was not a call for heat."

I'm wondering, is it possible this is just a bad flame sensor? Is that sensor a seasonal clean-it thing? All I've been doing since getting this system is putting a little compressed air on the electronics, in and around the burners etc. I could take this sensor out and see if it looks dirty but whole unit is 1.5 years old. Is it not worth the risk for something like the gas valve potentially leaking and should I just replace it? I DID check for smells of leaking and didn't smell anything. I simply restarted / power cycled the furnace and all was well.

Not interested in "call an hvac company" - where I live they are a long ways out on schedules even for emergencies. I already tried the big 3 in my area. No returned call or text from any of them. I think i can figure this out myself, just looking for advice from others with more experience.

1 Answer 1


flame sensor

(image: flame proving sensor, supplyhouse.com)

Yes flame sensors do get dirty over time and this leads to malfunction. Usually a dirty sensor would fail to detect the flame, rather than provide a false positive flame indication. The sensing rod can be cleaned with a wire brush, steel wool, etc.

Given that everything worked fine after the furnace was reset, we might be able to write this off as a fluke. If the problem had persisted, the comments below could help explain and troubleshoot it.

With this style of flame sensing, a small DC current flows from the sensing rod, through the flame, to the body of the furnace. A false positive flame indication is unusual and suggests the flame sensor's insulator is fouled with something electrically conductive, allowing that DC current to flow even when there's no flame present.

While you're cleaning the tip of the sensing rod, also inspect its insulator. Check for cracks or carbon tracks. Use water with a detergent, alcohol, or another suitable solvent to remove oil or other contaminant. Also examine the insulation along the wire that connects to the sensor: make sure it isn't cracked, shorting to a sharp edge somewhere in the furnace, etc.

You can try running the furnace with the flame sensor disconnected. The furnace should start the inducer (vent) blower, heat the igniter, open the gas valve, and start burning. Within a few seconds it should close the gas valve and shut down with a "no flame detected" error. If instead you continue getting the "flame sensed when there's no call for heat" error there may be a problem with your furnace control board.

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