Set Appliance State:

Mains Power Off
Isolate Mains Power to appliance
as per TOP's P4
Set Multimeter to Resistance Set meter to measure resistance
Remove the plug from the gas valve.

Measurement:

Measure the resistance across the terminals of the gas valve.
Resistances should be approximately as in the table below.
Resistance
Pin 1Pin 53.8 kohms

Reconnect the connector to the gas valve after this test.