Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance

Measurement:

Inspect the two harnesses between the gas valve and the PCB for good overall condition.
Check for continuity (zero resistance) of the white, black and blue wires running from the PCB to the Gas Valve.
Note: Connection to the gas valve differs depending on which type is fitted (see pictures on left).