Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove 6 wire connector from X6 on the PCB
Remove the wires from the air pressure switch

Measurement:

Measure the resistance of each of the wires running from X6
(pins 9 and 10) to the air pressure switch.
Expect continuity / zero resistance on each wire.
Also visually inspect the harness.
Is the resistance as expected and the harness in good condition?