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.
Note: There are different versions of this appliance, connections to the gas valve can be 2 x purple leads
or blue and purple leads, the gas valve can be either a SIT or PRETTLE/JUNKERS gas valve

Measurement:

Identify the gas valve fitted to the appliance (sit or prettle/junkers).
Measure resistance across gas valve connections on connector ST9.