Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect the brown wire from the air pressure switch common terminal
Measurement:
Measure the resistance between the normally closed terminal (white wire) and the common terminal (exposed terminal where brown wire was connected) of the air pressure switch.
Isolate as per TOPs P4
and refit as necessary