Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Disconnect grey leads from the overheat thermostat situated on flow pipe to the right of the heat exchanger

Measurement:

Measure resistance across the exposed terminals of the sensor.
Reconnect leads after test.
Note:
Expected resistance short circuit/zero resistance.
If fused leads are used resistance of leads will have to be taken into account of the reading.