Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect ST9 connector from the right hand side of the PCB.
If available use the Worcester test PCB to assist resistance test

Measurement:

Measure for resistance across pins 1 & 2 (blue) and
3 & 4 (purple) of ST9 connector.
Note.
Gas valve connection plugs can be fitted in any configuration.