Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Remove the rearmost ignition lead from the spark generator and position it just inside its slot so sparking can be observed (as shown in the image)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Allow the fan to run.
Does sparking occur?