Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove the orange and white wires from the air pressure switch noting their positions
Remove the orange and white wires from the air pressure switch noting their positions
Measurement:
Measure the resistance across the exposed terminals on the air pressure switch.
Is there short circuit resistance between C and NC terminals on the air pressure switch?
Reconnect wires after test orange wire to common terminal white wire to normally closed.
Is there short circuit resistance between C and NC terminals on the air pressure switch?
Reconnect wires after test orange wire to common terminal white wire to normally closed.
Isolate as per TOPs P4
and refit as necessary