Ducane package unit with gas furnace. Led flashes 3 times which indicates pressure switch stuck closed or open. I unhooked tubing and blew air into it, seems to be working OK, also I disconnected wiring and re-attached. This is the honeywell switch mounted on left side wall of compartment. I cycled unit several times but it kicks out after running about 1 minute. Led flashes immediately 3 times which leads me to believe there is a manual reset button somewhere but I'm not sure where it is. Please help ! Gas co. Service man was here on Sunday had it running several hours after he left but it tripped again. I think the pressure switch is defective and I have another service call in but I would sure like to be able to troubleshoot things myself like the old days. Thanks