Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove connector (blue wires) from the gas valve
Disconnect connector ST16 from the PCB

Measurement:

Check for continuity (zero resistance) of both of the blue wires running from the gas valve to pins 16 and 17 at ST16.
Check the harness and connectors for overall condition.