Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect connector from X4 on the PCB
Measurement:
Measure resistance across the white wires at the disconnected connector.
The resistance will vary according to the boiler temperature but should conform to the table below:
The resistance will vary according to the boiler temperature but should conform to the table below:
| Temperature °C | Resistance in kOhms |
| 20 - 30 | 12.0 to 8.5 |
| 30 - 40 | 8.5 to 6.0 |
| 40 - 50 | 6.0 to 4.2 |
| 50 - 60 | 4.2 to 3.0 |
| 60 - 70 | 3.0 to 2.2 |
| 70 - 80 | 2.2 to 1.7 |
Isolate as per TOPs P4
and refit as necessary