Trane XR90 pressure switch error

I'm receiving a pressure switch error code on my Trane furnace. This issue seems to be intermittent, but not getting frustrating.

To date I have removed the trap ( washed it out ), blown thru as many hoses as possible, and removed the intake and exhaust ( blowing each out with a dry vac.

The crazy issue is that the furnace will run perfectly for a few days, then start tossing the error code, again. A week ago, I removed the pressure switch vacuum line that goes to the burner box...the furnace fired and ran perfectly...which I assume, it should not have fired.

Anyone ever been confronted with an issue of this nature & what could possibly be the problem? Also, my CO detectors have not detected anything. Wouldn't they sound off if there was an issue with the heat exchanger?

Thanks for reading & look forward to responses.

Jim