Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Gain access to combustion chamber
DHW temperature set to MAX
Selector switch in Hot Water Position
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the fan run for at least 6 seconds?