Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove functional case door
Remove the orange leads from the sensor

Measurement:

Measure resistance across the terminals of the sensor.
The resistance measured will depend on boiler temperature, the higher the temperature the lower the resistance.
Resistance should be approximately:
14.0 kOhms at room temperature,
7.5 kOhms at 30 degrees C,
2.0 kOhms at 60 degrees C.