Set Appliance State:
On/Off Switch OFF
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove the blue wire from the air pressure switch
Remove the blue wire from the air pressure switch
Measurement:
Measure the resistance across the exposed terminal and the other terminal (black wire).
Reconnect wire after test.
Reconnect wire after test.
Isolate as per TOPs P4
and refit as necessary