Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4

Measurement:

Measure the resistance across the HW Sensor connector M10 connection 43 (Green wire) and connection 44 (Green wire) on the main PCB.
This should be in the range 1.5Kohms to 16Kohms.