Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Ensure boiler is not at its maximum operating temperature
Remove the connector plug from the CH temperature sensor (yellow wires)

Measurement:

Measure resistance across the exposed terminals of the sensor.
Replace connector plug after test
Note: The resistance measured will depend on the boiler temperature, the cooler the water the higher the resistance.