Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Disconnect 12 pin connector marked D18 to D29 (top L/H side) from main PCB 23

Measurement:

Measure resistance across connection D24 (yellow wire) and connection D23 (grey wire) at disconnected connector.
Resistance should be between 80-100 Ohms.