Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove connector from gas valve
Remove X3 from the PCB

Observation:

Test 1:
Measure the resistance across pins 3
(black) and 2 (blue) at connector X3.
Expected resistance greater than 1MOhm (or O.L).
Test 2:
Measure the resistance across pins 3 (black)
at connector X3 to a suitable earth point.
Expected resistance greater than 1MOhm (or O.L).