I have a ultra sx 80 furnace. Lately the furnace would work fine for a while and all of a sudden it will blow cold air instead of hot.
It usually happens when the heat is supposed to start. I can hear the exhaust fan working, I see the ignition lighting and then the pivot light goes on. The main burners never start and the furnace starts blowing cold air.
If I turn it off from the switch and wait about 15 minutes and turn it back on it will work fine for a while and then the same issue again.
I took the flame sensor out and cleaned it. The furnace worked for 3 days and then again the same issue.
What will be my next step?