Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove the connector from the DHW flow switch
Remove connector X4 from the PCB

Measurement:

Measure the resistance across each of the wires in the DHW flow switch harness.
Expect continuity / zero resistance on each test.
Also visually inspect the condition of the harness.