Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove blue and black leads from the return sensor
Remove blue and black leads from the return sensor
Measurement:
Measure the resistance across the exposed terminals of the sensor.
The resistance measured will depend upon the boiler temperature, the higher the temperature the lower the resistance.
The resistance should conform to the table below.
The resistance measured will depend upon the boiler temperature, the higher the temperature the lower the resistance.
The resistance should conform to the table below.
| Temperature °C | Resistance in kOhms |
| 25-35 | 9.5-6.1 |
| 35-45 | 6.1-3.9 |
| 45-55 | 3.9-2.7 |
| 55-65 | 2.7-1.9 |
| 65-75 | 1.9-1.4 |
| 75-80 | 1.4-1.2 |
Isolate as per TOPs P4
and refit as necessary