Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove leads from air pressure switch and remove connection X4 from PCB

Measurement:


Check the continuity and general condition of each of the two white leads in the harness.
Checking each lead in turn from air pressure switch to PCB connector.
Does each individual lead show continuity?
Expected reading short circuit / zero resistance.