Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect the NTC sensor from the harness
Measurement:
Measure the resistance between the thermostat on the heat exchanger flow pipe and a suitable ground point on the appliance (i.e. the return pipe).
Measure the temperature of the return pipe near the thermostat. The resistance varies according to the boiler temperature and should conform to the table below.
Measure the temperature of the return pipe near the thermostat. The resistance varies according to the boiler temperature and should conform to the table below.
| Temperature °C | Resistance in kOhms |
| 30 - 40 | 2.1 - 1.3 |
| 40 - 50 | 1.3 - 0.8 |
| 50 - 60 | 0.8 - 0.6 |
| 60 - 70 | 0.6 - 0.4 |
| 70 - 80 | 0.4 - 0.3 |
Isolate as per TOPs P4
and refit as necessary