Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect spades from DHW sensor
Disconnect connector X5 from the PCB

Measurement:

Check for continuity (zero resistance) between the ends of the two orange wires running from the DHW sensor to pins 7 and 8 at connector X5.
Inspect the harness and plugs for good overall condition