Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect Sensor Lead

Measurement:

Measure resistance across the Sensor Terminal and a suitable earth point.
The resistance will vary according to the boiler temperature.
The hotter the boiler the lower the resistance.
It should conform to the table below:
Temperature °CResistance in kOhms
30-402.1-1.3
40-501.3-0.8
50-600.8-0.6
60-700.6-0.4
70-800.4-0.3