Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove the brown wire from the air pressure switch
Remove the brown wire from the air pressure switch
Measurement:
Measure the resistance across the exposed terminal (common) on the air pressure switch and the N/O terminal (blue wire).
Reconnect the brown wire after test.
Reconnect the brown wire after test.
Isolate as per TOPs P4
and refit as necessary