Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector CN9 from the PCB

Measurement:

Measure the resistance (continuity) across the flow sensor harness leads.
Connections are;
CN9 pins 6 & 5 to the corresponding connection on the DHW flow sensor.
Expected resistance is continuity (zero resistance) across each wire of the harness.