Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Disconnect white leads from the overheat thermostat situated on flow and return header on the heat exchanger
Disconnect white leads from the overheat thermostat situated on flow and return header on the heat exchanger
Measurement:
Measure resistance across the exposed terminals of the sensor.
Expected resistance short circuit zero resistance. If fused leads are used resistance of leads will have to be taken into account of the reading.
Expected resistance short circuit zero resistance. If fused leads are used resistance of leads will have to be taken into account of the reading.
Isolate as per TOPs P4
and refit as necessary