Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the cover from the water pressure switch
This will just pull backwards down the wiring
Disconnect the wires from the water pressure switch
Note
Ensure that the system is pressurised before undertaking this test.

Measurement:

Measure the resistance between the exposed
terminals of the water pressure switch.