Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect gas valve plug from PCB (rightmost plug)

Measurement:

Measure the resistance across the black wire to
the blue wire next to it at the disconnected plug.
(if plug only has 1 blue wire next to the brown then use this.)
Resistance should be approximately 1.5kΩ.
Note:
Two types of gas valve lead may be fitted, refer to
the image that corresponds to what it is fitted on
the appliance you are working on.