I have a Coleman furnace model FG9D12020UP11B that cycles on correctly for heat but then appears to drop the flame (sometime during the heat cycle) but the furnace does not see that the flame is gone and continues to cycle cold air until I lower the thermostat below the temp or use the furnace shut off switch. Eventually I can turn it on and it will stay on through the entire heat cycle. This happens between 1 and every 5 times to 1 and ever 10 times the heat is called for and just started this heating season.
The furnace guy that came to look at it (emergency call guy) as part of my service contract the first time it happened said it was probably a dirty flame sensor, which was a metal rod that he cleaned with some steel wool. But it is still happening. He was the first to admit that "he isn't a gas furnace guy" which I understand as here in Maine almost everyone heats with oil. I'm reluctant to have the company come back even if they have a "gas guy" without some pointers or knowledge about what to check and what might be the cause. I don't want them to just start replacing everything if they don't have a reasonable idea what the cause could be.
I've woken up twice now to the house cold and the furnace blowing cold air for God knows how long...
So, if this was your service call and you just cleaned the flame sensor, what next?