CO is not a hydrocarbon. No hydrogen in it--just C & O. Those sniffers Will pick up aldehydes such as paraldehyde and acetyladehyde.

If this unit is LP, it can hold unbured gas for a long time after it was cycled off.

Needs to be inspected closely by a qualified technician who has a combustion analyzer and knows what to do with it. Yes, ANSI stds allow 800 ppm from gas ranges. And you just thought it was the tryptophan from the turkey making you sleepy at Thanksgiving!

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Keep the fire in the fireplace.