Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector from flow switch
Disconnect connector X6 from the control PCB

Measurement:

Check for continuity of the harness between the flow switch and the control PCB:
Red to Red (pin 4 on J6).
White to White (pin 5 on J6).
Blue to Blue (pin 6 on J6).
Also inspect the overall
condition of the harness.