Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
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.
Reconnect the wires to the Water Pressure Switch after this test and replace its cover.