Set Appliance State:

Mains Power Off
Turn Hot Water Tap On
DHW Temperature Selector set to MAX
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary.
Can the fan be heard to run for at least 5 seconds?
If you cannot hear the fan running:
Isolate Mains Power to appliance as per TOP's P4
Gas Off
Remove combustion chamber and repeat test to visually confirm if fan is running or not.