Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect inline connect from flow sensor
Disconnect ST16 plug from PCB

Measurement:

Measure resistance of each wire of the harness to the DHW flow switch.
Black pin 3 on ST16 to black - inline connector.
White pin 2 on ST16 to white - inline connector.
Green pin 1 on ST16 to green - inline connector.
Expect zero resistance on all tests.
Also inspect the overall condition of the harness for damage.