Set Appliance State:
Mains Power Off
Gas Off
Set Multimeter to Resistance 
CH Demand
Pre-heat switch OFF
CH switch ON
Ch temperature selector to maximum position
CH switch ON
Ch temperature selector to maximum position
Gas On
Mains Power On
Measurement:
Allow boiler to fire and system to heat to approximately 70 degrees C.
Mains power OFF.
Disconnect plug from CH temperature sensor (yellow wires).
Measure resistance across the exposed terminals of the sensor.
Replace plug after test.
Note: It may be necessary to reduce the heating load by turning off some radiators to reduce the time taken for this test.
Mains power OFF.
Disconnect plug from CH temperature sensor (yellow wires).
Measure resistance across the exposed terminals of the sensor.
Replace plug after test.
Note: It may be necessary to reduce the heating load by turning off some radiators to reduce the time taken for this test.
Isolate as per TOPs P4
and refit as necessary