Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove the White wires from the CH Sensor
Remove the White wires from the CH Sensor
Measurement:
Measure the resistance across the terminals of the sensor.
The resistance measured will depend on boiler temperature (the colder the water the higher the resistance) and should lie between 10 and 13kOhms, with the CH water temperature of 20-25 degrees C.
The resistance measured will depend on boiler temperature (the colder the water the higher the resistance) and should lie between 10 and 13kOhms, with the CH water temperature of 20-25 degrees C.
Isolate as per TOPs P4
and refit as necessary