Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector X8 (11way connector, far right of PCB) from the PCB and the connector from the DHW flow turbine

Measurement:

Measure the resistance across the each of the wires in the disconnected connectors from the turbine end to the PCB end in turn.
Red = X8 pin 10
Blue = X8 pin 9
Black = X8 pin 8
(Pin 11 of X8 is an Orange wire)
Inspect the leads general condition.