Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove the connector J2 from the PCB
Remove the connector from the DHW flow switch

Measurement:

Measure the resistance of each of the wires in the harness.
Pink to Pink
White to White
Grey to Grey
Expect zero resistance on each of the wires in the harness.
Also inspect the condition of the harness visually.