I'm totally stuck on this one. I have a 12yr old Comfortmaker RPJ2. It was blowing cold air, so I replaced the thermostat, and had a company in to look at it. Apparently, it was so dirty, it was overheating and shutting down, leaving the fan blowing cold air. Ok, fine... They replaced the "high limit" (sensor?), and the flame sensor, and we had the furnace and vents cleaned. It worked fine for 3 days... Now it seems to intermittently blow cold air again. The only way to get heat again is to shut the heat of at the thermostat, then turn it back on again. Diagnostic is usless. No flashing lights, nothing.. We had the repair guys come back, and he couldn't find anything wrong with it. (Of course, it worked fine when he was here... figures..) I've already spent over $900 on this fiasco, and I still have the same problem as I started with. Any ideas?
Since the problem was masked when you had the service tech out, I must assume it works most of the time. It sounds like it doesn't ligh some times and goes through tree tries and fails, causing the furnace fan to come on and the ehat to go off. If it is a 90% furnace check the grommett located ont he wires going up to the hot surface ignition. If it's bad take a littleplumbers putty and fill in around the wires. Let me know if it's not a 90% furnace and I'll continue to work the problem.