Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect wires from air pressure switch
Measurement:
Measure the resistance between the common and normally closed (front and middle spades) at the air pressure switch.
Expect continuity (zero resistance).
Expect continuity (zero resistance).
Isolate as per TOPs P4
and refit as necessary