Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connectors CN203 from the PCB and from the DHW flow sensor

Measurement:

Measure the resistance of the wiring harness from connector CN203, pins 1 (red lead), 2 (white lead) and 3 (pink lead) to the corresponding lead of the removed flow switch connector.
Expect continuity / zero resistance on each wire.