Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect blue leads from the return sensor
Disconnect blue leads from the return sensor
Measurement:
Measure resistance across the exposed terminals of the sensor.
Resistance measured will depend upon the temperature of the appliance.
The cooler the appliance the higher the resistance and should conform to the table below.
Resistance measured will depend upon the temperature of the appliance.
The cooler the appliance the higher the resistance and should conform to the table below.
| Temperature °C | Resistance in kOhms |
| 20-30 | 10.0-9.0 |
| 30-40 | 9.0-6.5 |
| 40-50 | 6.5-4.5 |
| 50-60 | 4.5-3.1 |
| 60-70 | 3.1-2.1 |
| 70-80 | 2.1-1.5 |
| 80-90 | 1.5-1.0 |
Isolate as per TOPs P4
and refit as necessary