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 inline connector to flow switch

Measurement:

Check for continuity (zero resistance) between the wires running from the flow switch to pins 6 (red), 7 (pink), and 8 (black) at connector X7.
Inspect the harness and plugs for good overall condition.