I have had twenty some years experience in the hvac field, but havent done it for fifteen yrs. I looked at a buddies furnace today, trane XE 90, started the cycle, everything ran fine. Good ignition, good flame, etc. When the main blower started, I noticed the draft inducer seemed to pick up rpms as well, didnt know if that was part of the design. Shortly after the main blower came the lights started flashing three times"pressure switch error". But the stumper was that the furnace kept running, it didnt kill the burner. I cycled the t stat several times waiting each time for the furnace to stop, same thing each time, error code, but flame and blower stay on. Apparently my expertise is somewhat dated, any suggestions would be greatly appreciated.