Deeep breath and.......I have a ~8 year-old York Diamond 90 furnace which will not heat. The ventilator fan starts and runs, but after a few seconds the ignition module (White Rodgers 50A55-843 Universal) shows that the draft pressure switch is still open. It is the two-wire, two-tube type, Tridelta (manuf.) p/n PPS10129-2764 or York p/n 024-27638-000. I took the positive pressure tubing off and blew into it; the switch will activate with very little pressure (nameplate is 1" w.c. ). If I do this during startup, the ignitor will glow after 30 seconds, so at least the microswitch is good. The ignition module's timer is set at 30 sec. INcidentally, I can see the pressure switch's diaphram move about half-travel, so it's trying to get there, but either the bellows has gone stiff, or there's some leak somewhere. So, not being able to easily test the pressure switch, I took off the fan and inspected it. I've replaced this fan about two years ago when I found the rotor locked-up at the beginning of the heat season. This year, I squirted WD40 on the end shaft about a month ago hoping to avert this problem. So the motor turns fine. Also, I checked the negative pressure port which is tubed to the neg. port of the pressure switch. It's not blocked. Nor are any other tube fittings on other ends of the tubing runs, including up to the gas valve. I looked online and could find a 0.9" w.c. switch which seems to be identical, if not for the setpoint difference. I feel that this 10% difference will not have any detrimental effects. Shoot, if a small handful of leaves gets in the vent, there goes 0.1" drop. But when I asked for opinions on another DIY site about this, I was pummeled into submission; they said don't do it. I still don't see a major problem with that slightly lower setpoint, but nontheless gave up that idea. I tested the delta-P across the switch with a piece of tubing, two tee-pieces, to create a manometer. When running the ventor fan, the differential pressure was 'right at' 1 inch of water. So I bought a new switch. That didn't solve the problem; the ignition control module still reported that the pressure switch was open. So now I'm going to test the ventor motor run capacitor and also put the shop-vac to the condensation drain line - I believe it's clogged because there's water in the corners of all horizontal sheetmetal, and they're rusting even though it's galvanized. Last night I vacuumed the condensate line out and man, out came some junk and quite a bit of water. That probably explains the water leak inside the furnace. I tried to start again after that cleanout, still the flashing LED indicating a stuck pressure switch. So now, I'm going to remove the condensate pan so I can see if the water did any other damage or if the airway is leaking somewhere preventing the ventor from providing 1" pressure drop. I see what appears to be a pressure test port on the pan access panel. If that's it's function, does anyone know what the pressure should be? Many thanks in advance for ANY advice!!