I have a 20+ year-old Carrier gas fired furnace that is logic controlled (that is, it has an electronic control unit in the top of the fan compartment). For some time, instead of starting the blower fan as it should after the burner has come up to a certain temp, the burner just runs until it (apparently) shuts down from over-temp. THEN the fan immediately comes on and runs for about a 90 seconds. This is really inefficient! I know the unit is old, but I'd like to squeeze another season or two out of it. Since the problem obviously lies with the control unit or with a sensor, I'm thinking of replacing all of the sensors and the control unit. This would be expensive, but this seems to me to be what a service tech would probably do, and then I'd be out the bucks for the service call AND the new hardware.

Any thoughts about how to fix this irritating type of failure? -GA

Replace sensor first.