I have what I have found by searching other forums is a fairly common problem with a Trane XE90 furnace, I get a code when it dose'nt ignite (3flashes of a led) that shows the pressure switch is stuck open. Some times the furnace lites fine with no problem. Other times it will run the exhaust blower and start the igniter and drop out before the gas lites. Sometimes the exhaust blower will run and run and finally it will ignite after many minutes. There seems to be no pattern to it. Suggestions in other forums have been possible water in the system, clogged vent, locked up rotor in the exhaust blower. I have checked all of these with no luck. Since it happens sporatically I am sure it could be hard to determine the cause. I have a friend in the HVAC bussiness and it has him stumped.