Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Disconnect both the Air Pressure Tubes from the Air Pressure Switch noting their positions.
Measure the resistance across the Purple wire connection 8
(Pressure Switch connection C) and the Red/White wire
connection 9 (Pressure Switch connection NC) on the PCB.
Note:
Test carried out on PCB (control box) not Air Pressure Switch.
Reconnect the Air Pressure Tubes to the Air Pressure Switch after test.
Measure the resistance across the Purple wire connection 8
(Pressure Switch connection C) and the Red/White wire
connection 9 (Pressure Switch connection NC) on the PCB.
Note:
Test carried out on PCB (control box) not Air Pressure Switch.
Reconnect the Air Pressure Tubes to the Air Pressure Switch after test.
Isolate as per TOPs P4
and refit as necessary