Thermostate calls for heat, heat sensor gets red hot, pilot lites, but main burners doesn't. Then fan starts circulating cold air trying to bring the temp to the set temp on the thermostate. I turn unit off to stop the cold air circulating and when the repair guy arrived and we turned everything on again. It worked!!! They have been back twice and I am waiting for them again today for the 3rd straight day. I was hoping to have it fix by replacing either the gas valve and/or the heat sensor, but judging by what I have read here, now I am not too optimist. Don't buy Comfortmaker if you have any saying on it. It was what my builder put on my new house. Just for kicks, anybody with this problem out there? This is the 1st time it happens going into my 4th year.