Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove connector CN9 from the PCB
Remove both wires from the water pressure switch.

Measurement:

Measure the resistance of each of the 2 wires coming from connector CN9 running to the system pressure switch.
Expect continuity / zero resistance.
Also visually inspect the condition of the harness.