Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove functional case door
Remove the grey wires from the dry fire sensor

Measurement:

Measure resistance across the terminals of the sensor.
The resistance measured will depend on temperature, the higher the temperature the lower the resistance.
Resistance should be approximately:
14.0 kOhms at room temperature,
7.5 kOhms at 30 degrees C,
2.0 kOhms at 60 degrees C.