Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove Harness from Gas Valve
Remove connector X6a from PCB

Measurement:

Test resistance of the wire. The Neutral splits in two at gas valve plug so must be tested on both.
The Gas Valve connector is numbered 1 to 5 from top to bottom.
3 & Live (Brown on X6a Plug).
then 1 & Neutral (Blue on X6a Plug).
and 4 & Neutral (Blue on X6a Plug).
Expect short circuit on all tests.