Set Appliance State:

Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Unplug both M3 & M5 and test at plug
Turn on HW Tap

Measurement:

Measure short circuit resistance between connector M3 connection 9 (Red) and connector M5 connection 19 (Orange)
Turn off HW Tap.
Measure open circuit resistance between connector M3 connection 9 (Red) and connector M5 connection 19 (Orange)
Replace plugs after test.