Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect plug (red leads) from primary sensor
Measurement:
Measure the Resistance across the two sensor terminals, also measure the approximate temperature at the sensors location.
The resistance will vary according the boiler temperature but should conform approximately to the table below at the appropriate temperature to ensure it is within range and in calibration
The resistance will vary according the boiler temperature but should conform approximately to the table below at the appropriate temperature to ensure it is within range and in calibration
| Temperature °C | Resistance in kOhms | 20 | 10 |
| 30 | 6.6 |
| 40 | 5.8 |
| 50 | 3.1 |
| 60 | 2 |
| 70 | 1.5 |
| 80 | 1.1 |
Isolate as per TOPs P4
and refit as necessary