Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Carefully remove wires from thermistor.
Check resistance of thermistor is between 1K and 30Kohms.
A reading of approx 12Kohms will be obtained if boiler is cold.
This corresponds to a temperature of 20 deg C.
Carefully replace leads after test.
Check resistance of thermistor is between 1K and 30Kohms.
A reading of approx 12Kohms will be obtained if boiler is cold.
This corresponds to a temperature of 20 deg C.
Carefully replace leads after test.
Isolate as per TOPs P4
and refit as necessary