Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Gain access to PCB (use Insulation Mat as required)
Disconnect both ignition leads from the spark generator and make safe
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary.
Shortly after the fan starts to run can sparking be heard from the PCB?