Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Unplug M5 from Control Box & test at plug

Measurement:

Check for short circuit resistance between connector M5 connection 24 (COM Red/Black) and connection 21 (NC Blue/Black).
Check for open circuit resistance between connector M5 connection 24 (COM Red/Black) and connection 23 (NC Purple).
Replace plug M5 after test.