Set Appliance State:
| Isolate Mains Power to appliance as per TOPs P4 |
Set Multimeter to Resistance 
Remove the Brown wire and the Black wire from the CH Thermistor
Measurement:
Measure the Resistance across the terminals on the Thermistor.
The resistance measured will depend on boiler temperature and should lie between 500 ohms and 1500 ohms, the cooler the water the lower the resistance. For boiler to operate at its maximum set gas rate resistance should be between 500 ohms and 1000 ohms.
Replace leads after test.
Note:
If this test has been called after failing burner modulation the resistance of the Thermistor should be towards the top end of its range i.e. water temp 60 deg C 1300 ohms water temp 80 deg C 1490 ohms.
The resistance measured will depend on boiler temperature and should lie between 500 ohms and 1500 ohms, the cooler the water the lower the resistance. For boiler to operate at its maximum set gas rate resistance should be between 500 ohms and 1000 ohms.
Replace leads after test.
Note:
If this test has been called after failing burner modulation the resistance of the Thermistor should be towards the top end of its range i.e. water temp 60 deg C 1300 ohms water temp 80 deg C 1490 ohms.
Isolate as per TOPs P4
and refit as necessary