Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove electrical connector to DHW temperature thermistor
& connector CN9 from the PCB

Measurement:

Check for continuity (zero resistance) between each of the wires running from the DHW thermistor plug to pins 3 and 4 at CN9 on the PCB.
Check the harness and plugs for good overall condition.