Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Expose PCB by removing plastic control box cover.
Remove Air Pressure Switch connector (D19-D24) from PCB

Measurement:

Test 1:
Check resistance between terminal marked COM
(Red/Black wire) and terminal marked N.O. (White wire)
on air pressure switch.
Resistance should be open circuit.
Test 2:
Check resistance between terminal marked COM
(Red/Black wire) and terminal marked N.C. (Brown wire)
at air pressure switch.
Resistance should be short circuit.
Replace connector.