I had my preasuure switch replaced which fixed my furnace wich would not ignite. In the process the tube from the switch to the exhaut plower had a slice in it letting in air. Ironically last night I smelled a plastic odor in the house and the unit would not turn on. When I take the door off and maually strt the furnce in fan mode only, it shorts out. Seems to me the exhaust fane may have burned out. Could the cut in the preasure switch line caused this. And what is a way to confirm the fan is shot?