Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove / loosen right hand side panel
Disconnect the connector from the water pressure switch

Measurement:

Measure the resistance between pins 8, 9 and 10 (brown, blue and black wires on connector CN31) to the corresponding pins of the water pressure switch harness.
Expect continuity / zero resistance on each of the wires.
Also visually inspect the harness for good general condition.