We have a majic chef g66-150-1 furnace with a manufacturing date of 1978. We replaced the thermocoupler last year and when we turned on the furnace this year it will not shut down. It ran for over 2 hrs the other night and I had to shut the thermostat down all the way then it took about 5 minutes to shut off.

Is the main burner on the whole time or is it just the fan?