Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the orange lead from the terminal marked C and the white lead from the terminal marked NC on the air pressure switch
Disconnect the orange lead from the terminal marked C and the white lead from the terminal marked NC on the air pressure switch
Measurement:
Measure resistance across the exposed terminals marked C and NC of the air pressure switch.
Expected resistance closed circuit.
Expected resistance closed circuit.
Isolate as per TOPs P4
and refit as necessary