Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Disconnect CN200 from the ignition PCB
Refit wires to APS if you have not already done so

Measurement:

Measure the resistance between pins 1 and 3 at the disconnected plug.
Resistance should be short circuit (zero resistance)