Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the Yellow spaded wires from the Thermostat Sensor.
Measurement:
Measure the resistance across the unconnected terminals of the sensor.
The sensor resistance should be approximately 1 MOhm at 20 °C but will vary depending upon its temperature.
The higher the boiler temperature the lower the resistance.
The sensor resistance should be approximately 1 MOhm at 20 °C but will vary depending upon its temperature.
The higher the boiler temperature the lower the resistance.
| Temperature °C | Resistance |
| 20 | 1.2 Mohms |
| 30 | 780 kohms |
| 40 | 480 kohms |
| 50 | 296 kohms |
| 60 | 183 kohms |
| 70 | 115 kohms |
| 80 | 70 kohms |
Isolate as per TOPs P4
and refit as necessary