Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the plug of the DHW temperature sensor from the PCB it is adjacent to the low voltage DC terminal strip marked ST 18 on the PCB

Measurement:

Measure the resistance across the blue and brown connections on the disconnected plug.
Resistance will vary according to the temperature of the water.
The lower the temperature the higher the resistance.
The resistance should conform to the table below:
Temperature °CResistance in kOhms
20-3014.0-11.8
40-509.0-7.5
60-705.7-4.5

Note:
Expected temperature should not exceed 60 degrees C.