Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Disconnect connector from ST9 on the PCB
Measurement:
Measure resistance across connections 17 and 18 yellow leads at the disconnected connector .
The resistance will vary according the boiler temperature but should conform to the table below:
The resistance will vary according the boiler temperature but should conform to the table below:
| Temperature °C | Resistance in kOhms | 20-30 | 8.0-16.0 |
| 30-40 | 6.0-10.0 |
| 40-50 | 3.9-8.0 |
| 50-60 | 2.5-5.5 |
| 60-70 | 1.5-3.0 |
Isolate as per TOPs P4
and refit as necessary