Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both green wires from the air pressure switch
Remove the ST16 plug from the PCB connection

Measurement:

Measure the resistance of each of the two green wires from the ST16 plug to the two spade connectors.
Inspect the harness for good overall condition.