Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect the Blue wires from the HW Inlet Sensor
Disconnect the Blue wires from the HW Inlet Sensor
Measurement:
Measure the resistance across the Sensor.
The resistance measured across the Sensor should be steady or only slowly changing.
The resistance will vary depending on boiler temperature and should be approximately 64 K ohms at 10 degrees C and 3 K ohms at 80 degrees C.
The lower the water temperature the higher the resistance.
Replace the leads after test.
The resistance measured across the Sensor should be steady or only slowly changing.
The resistance will vary depending on boiler temperature and should be approximately 64 K ohms at 10 degrees C and 3 K ohms at 80 degrees C.
The lower the water temperature the higher the resistance.
Replace the leads after test.
Isolate as per TOPs P4
and refit as necessary