It will work fine for a day, a week,. Anyway it will drop to mid 60s on Tstat (cold for us, set on 75) and won't start. My Tstat is located in the hall with no vents nearby. Ill flip the switch to ac and ac will work fine. Fan comes on and all. Flip it back to heat and hear the click for the call for heat but the system will not start unless I turn the switch off and back on. It can work for 1 cycle, 10 cycles, but eventually it will act up again. 2 friends with HVAC experience (not experts) checked it out and said flame was a good color, flue was nice and clear. Only code I am seeing right now it 12. Anyone have any clues. I really can't afford a full on service call with Christmas coming up.

If it vents with plastic pipe, most likey you need to clean your drain out.

Vent with plastic?? Do you mean the intake/exhaust. If so it's a double pipe galvanize. One inside the other. If it's a water drain ill have to double check for location. Just in case I changed the Tstat a couple months ago from the OG to a Honeywell. Same issue but at least I hooked it up right haha.

Try cleaning the flame sensor it's a metal rod that sets in the flame of one of the burners it is usually held in by one 1/4 inch screw

Cleaned the flame sensor before. Didn't effect it. This is something to do with the signal not making it from the tstat to the furnace or the furnace not interpreting it correctly. I just don't know enough about the inner workings or wiring of it to know how to diagnose and fix. I do appreciate the ideas and I hope they keep coming. Eventually something will lead me to the culprit.

Unplug the circuit board and clean the contacts between board and wire harness. Dust collects on this and causes intermittent problems. If this doesn't fix problem you may need a new circuit board.

Hi John, did you get this figured out. I am having the same problem recently and don't know what to do.

John or Ryan...
Did you get the fix for this irritating problem? I have the same problem and have to keep turning the furnace switch off and on to reset.