Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect right hand plug from the PCB

Measurement:

Check for continuity of the harness between the red (20), brown (21) and orange (22) wires at the disconnected plug to the corresponding wire at the air pressure switch.
Visually inspect the harness for any signs of damage or wear.