Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove all three wires from air pressure switch
Remove all three wires from air pressure switch
Measurement:
Measure open circuit resistance across the COM and NO terminals.
Measure short circuit resistance across the COM and NC terminals.
Replace all three wires as follows:
NC White wire
NO Black wire
COM Red wire
Measure short circuit resistance across the COM and NC terminals.
Replace all three wires as follows:
NC White wire
NO Black wire
COM Red wire
Isolate as per TOPs P4
and refit as necessary