Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect the blue lead from the air pressure switch
Measurement:
Measure the resistance between terminal marked N/O and terminal marked COM (black lead) on the air pressure switch.
(Resistance should be open circuit.)
(Resistance should be open circuit.)
Isolate as per TOPs P4
and refit as necessary