Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the three wires from the Air Pressure switch noting their positions.

Measurement:

Check resistance between Air Pressure Switch COM (Red/Black) and NC (Brown) is a short circuit.
Replace all three wires back on Air Pressure Switch as follows:
COM - Red/Black
NC - Brown
NO - White