Pressure Switch stuck open

I have a Intertherm (Nordyne) G5RC series High Efficiency Upflow Condensing Furnace.
I get a fault condition of 2 flashes (PS stuck open). The motor will run continually (literally 24/7) while it blinks. Sometimes when I remove the outer housing door, replace the door the unit will run through all the checks and the furnace will fire up for a single cycle. Meaning heat flows through the house until reaching set temp at thermostat. But the next time the thermostat tells it to fire up again, I'll get the fault.
The condensation tubes (that I can see) are not clogged.
I'm good with cars, lol, and generally good with my hands, but i don't have a troubleshoot manual and know nothing about heating furnaces.
Any and all help is appreciated!

Thanks,
Butch

 

Additional info..
Live in MA, so def cold and heater gets used alot. Its a gas furnace, that was added about 8 years ago when the original owner added on to the house and put this unit in to heat the new addition.

Hope this helps get an answer

 

Rather than stuck open, it sounds to me that since it can operate (when reset), it therefore has the ability to and does therefore "close"(make current flow) - but then goes open in the cycle, rather than stuck open.

What blower is running 24/7?: The inducer fan (the exhaust gas venting fan)?

Do you have a mulitmeter to use for testing?

The test would be to see where in the 24 volt safety circuitry it loses power. IF the pressure switch was or became "open", then 24 volt power would be coming into it, but not leaving it.