Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect plug from CH sensor (white and blue leads)
Measurement:
Measure resistance across the exposed terminals of the sensor.
The resistance measured will depend on the boiler temperature but should conform to the table below.
The resistance measured will depend on the boiler temperature but should conform to the table below.
| Temp °C | Resistance in kOhms | 20 - 30 | 8 to 16 |
| 30 - 40 | 6 to 10 |
| 40 - 50 | 3.9 to 8 |
| 50 - 60 | 2.5 to 5.5 |
| 60 - 70 | 1.5 to 3.0 |
Isolate as per TOPs P4
and refit as necessary