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 CH Sensor connector M10 connection 45 (Red wire) and connection 46 (Red wire) on the main PCB.
This should be in the range 1.5Kohms to 16Kohms.