Set Appliance State:
Mains Power Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the Control Box (PCB) multiplug (connections marked 1 to 14).
Disconnect and remove the Pressure Sensing Tubes from the Air Pressure Switch, noting their locations.
Test at the disconnected plug and measure the resistance across the Purple spaded wire connection 5 and the Red/White spaded wire connection 12.
NOTE:
Test carried out at disconnected plug NOT at PCB.
Replace plug after Test.
Replace Air Pressure Sensing Tubes in correct locations.
Disconnect and remove the Pressure Sensing Tubes from the Air Pressure Switch, noting their locations.
Test at the disconnected plug and measure the resistance across the Purple spaded wire connection 5 and the Red/White spaded wire connection 12.
NOTE:
Test carried out at disconnected plug NOT at PCB.
Replace plug after Test.
Replace Air Pressure Sensing Tubes in correct locations.
Isolate as per TOPs P4
and refit as necessary