Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector CN10 from the PCB
Remove wires from pressure switch

Measurement:

Measure the resistance between the each of the black wires, in turn, running from CN10 on the main PCB to the water pressure switch.
Expect continuity (zero resistance) on each test.
Also visually inspect the harness visually for general condition and any damaged insulation.