Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector from DHW thermistor

Measurement:

Check for continuity (zero resistance) of the blue and black wires running from the disconnected plug to X7 on the PCB.
Blue = Pin 1.
Black = Pin 2.
Inspect harness and connectors for good overall condition.
Note:
Wires may be multicoloured.