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 between the green wire at flow switch connector and the green wire (2) on ST16.
Expect zero resistance.
Also inspect the overall condition of the harness for damage.