Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove the purple wires from the CH sensor

Measurement:

Measure the resistance across the terminals on the sensor.
The resistance measured will depend on boiler temperature.
Temperature °CResistance in kOhms
10-2010.0-6.0
30-403.0-2.5
50-601.8-1.6
70-801.0-0.5