Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove both spade connectors (green wires) from APS.
Remove connector ST16 from PCB.

Measurement:

Measure the resistance of each of the green wires running between ST16 and the APS spade connectors.
Expect continuity / zero resistance.
Also visually inspect the condition of the harness.