Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect connector at gas valve

Measurement:

Check for continuity between the blue and brown and neutral wires of the gas valve plug and the rightmost brown (pin 6) and rightmost blue
(pin 7) wires at X6b connector at the PCB.
Also inspect the overall harness condition.
Note:
If a white board is fitted connector is in same location but is called X6 instead of X6b.