Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove brown and green plugs from the gas valve
Disconnect connector ST16 from the PCB
Remove brown and green plugs from the gas valve
Disconnect connector ST16 from the PCB
Measurement:
Test 1:
Check for continuity (zero resistance) of both of the brown wires running from the gas valve to pins 12 and 13 at ST16.
Check the harness and connectors for overall condition.
Test 2:
Check for continuity (zero resistance) of both of the green wires running from the gas valve to pins 19 and 20 at ST16.
Check the harness and connectors for overall condition.
Check for continuity (zero resistance) of both of the brown wires running from the gas valve to pins 12 and 13 at ST16.
Check the harness and connectors for overall condition.
Test 2:
Check for continuity (zero resistance) of both of the green wires running from the gas valve to pins 19 and 20 at ST16.
Check the harness and connectors for overall condition.
Isolate as per TOPs P4
and refit as necessary