15 year old York GY9S furnace won't fire due to pressure switch stuck open. It has two pressure switches. One switch connects to the collector box & is set to close at -.40 inH20. I got -2.20+/- inH20 manometer reading on this switch so it should close. The other switch has two ports & one connects to the inducer motor assembly & the other connects to the burner box. This switch is set to close at -1.50. Manometer reading on the connection to the inducer assembly was -2.19+/- so it should close, but the manometer reading on the connection to the burner box was only -.60 so this one is staying open. This is a brand new pressure switch by the way. I prematurely ordered it as soon as I saw the pressure switch stuck open error. Wasn't until I took readings with a manometer that I realized the switch wasn't the problem. I then ruled out easy stuff like kinks/blockages in the pressure switch tubing & found no issues. Disconnected condensation lines/trap & confirmed they're all clear by flushing with water (I primed the trap before reinstalling). Removed inducer motor assembly & confirmed no blockages in the housing/cage. Furnace has concentric intake/exhaust. With inducer out, confirmed exhaust venting is clear with camera scope. Removed rubber coupling joining intake pipe inside furnace housing (pic here https://i.stack.imgur.com/J2FYP.jpg) to gain access in both directions with camera scope & confirmed no blockages in intake pipe in direction of burner box & in direction of intake from outside. Could not see the intake cap clearly & don't have easy access to the second story to check it up close, so to rule out a blockage there, I ran the furnace with the rubber coupling joining the intake pipes off so the furnace will pull intake directly from the attic, but it still errored out with the same switch stuck open. Manometer reading on the connection to the burner box with the furnace intaking air from the attic was -.32 so even lower than the -.60 when connected to the intake normally from outside.
After eliminating blockages in the switch tubing, condensation line, inducer motor housing, intake/exhaust, I would think the only possible causes could be the inducer motor going bad & just not pulling enough of a draft or there's a crack in the heat exchanger. However, if there's a crack in the heat exchanger, shouldn't I also be getting low manometer readings from the switch connected to the collector box causing this switch to also not close and not just the one connected to the burner box? I got my camera scope into the heat exchanger a bit through the burner holes & into the inside of the furnace through the limit switch hole & took some photos (https://i.stack.imgur.com/H4ODF.jpg). You can see some rust lines along the pipes at what's probably the solder joints? Is this normal wear & tear or should I be concerned? I couldn't see any obvious cracks in any of the other pictures. The Fasco A225 inducer motor is fairly new having only been installed about 11 months ago when the original died. I tested the resistance of the inducer motor & only got 6.1 ohms with my meter set to the 200 ohms scale. Researching online, seems like the typical resistance for an inducer motor should be between 20-80 ohms? Is my inducer motor basically dying since I only read 6.1 ohms with my meter? Not sure if my readings are accurate because my motor only has two black wires in the main power plug to the motor & not the red, black, white that I'm coming across online when reading about testing inducer motor resistance. Is there anything else I should be looking at that could cause the stuck open switch? Thanks!