Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove connector CN9 from PCB
Remove wires from flow overheat stat.

Measurement:

Measure the resistance of each of the 2 wires running from connector CN9 to the flow overheat stat.
Expect continuity / zero resistance.
Also visually inspect the condition of the harness.