I have a 6 year old comfortmaker furnace that has stop heating. The exhaust fans runs all the time but the burner goes out after a couple of minutes. I've had repair men out and it seems every time they arrive, the heat starts working fine again. This usually happens once every year but this year, it's been two days now. When the repair man arrived, he found ice on the intake vent and after clearing it, the pressure increased and the system began working. After they left, the system shut down again. The motor on the exhaust fan is hot, and is registering 9 amp but should be 8 amps. I've changed the pressure switch, the intake pipes are 3 inches and I'm wondering if the motor could be the problem?

Comfortmakers are notorious for being sensative to icing, as well as incorrectly pitched vent/intake piping. Have the service tech check the pitch of the pipes and make sure there are no sags or low spots in the runs. A small amount of standing water in the intake pipe will shut it down...