Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector from the DHW flow turbine

Measurement:

Check for continuity (zero resistance) of each of the
3 wires running between X6 and the disconnected plug.
Red = X6 Pin 4
Black = X6 Pin 5
Blue = X6 Pin 6.
Inspect harness and connectors for good overall condition.
Note:
Wires may be multicoloured.