Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect 1 wire from the air pressure switch
Measurement:
Measure the resistance between the common and normally open terminals (left and right hand side spades) of the air pressure switch.
Resistance should be open circuit.
Resistance should be open circuit.
Isolate as per TOPs P4
and refit as necessary