We just moved into a house with a York Affinity 9s furnace. The unit turns on but does not blow heat, LED flashing red 3 times. There is a grey hose that goes from the heater box? To the pressure switch that if I remove and put back on after a few seconds allows the heater to start up. The furnace will work fine for a few hours but then the problem returns. Do I need to replace the pressure valve or is it something else?

Look on the inside of the blower door to see what 3 flashes mean.

3 red says pressure switch stuck open. But why would it work when I pull off the hose? If I blow, (or suck depending on which hose I choose), I can hear the pressure switch click.

Because the pressure switch is stuck open and when you remove the pressure hose you clear the problem. You will need a meter to service the unit to determine why the switch is not activating.

What type of meter? And is this something I can do myself, I have done a lot of renovating around the house.

Thanks for the help

Furnaces use vacuum and pressure lines in combination with pressure/vacuum activated switches to initiate some actions that the circuit board requires to allow the unit to start and run safetly. These are all considered safety devices and should be checked by a professional who know what they are doing.

If you have a factory service manual for your exact unit and can read and comprehend the meaning of the tests then if you have the proper meter I am sure you could test the unit and come to a logical conclusion as to its defective part (s).

The meter you need to start with is a manometer something like this

MANOMETER DUAL INPUT : Grainger Industrial Supply

OR this

Digital Manometer

Naturally having a electric test meter will be necessaty for checking any electrical malfunction.