Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect plug from water pressure switch

Measurement:

Check for continuity (zero resistance) between the ends of the black, red and pink wires running from the water pressure switch to pins 1, 2 and 3 of connector X7 at the PCB.
Inspect the harness and the plugs for good overall condition.