Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X7 from the PCB
Disconnect plug from flow switch

Measurement:

Check for continuity (zero resistance) between the wires running from the flow switch to pin 7 (pink) and 8 (black) at connector X7.
Inspect the harness and plugs for good overall condition.