Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Remove the Red wires (1) from the DHW thermistor and measure the resistance across the terminals on the sensor.
Resistance measured should be between 3.5 kOhms and 33 kOhms.
Note:
The resistance measured will depend upon the temperature of the DHW, the colder the water the higher the resistance.
With DHW at approximately 20-25 deg C the resistance should lie between 10-13 kOhms.
Resistance measured should be between 3.5 kOhms and 33 kOhms.
Note:
The resistance measured will depend upon the temperature of the DHW, the colder the water the higher the resistance.
With DHW at approximately 20-25 deg C the resistance should lie between 10-13 kOhms.
Isolate as per TOPs P4
and refit as necessary