Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Gain access to the combustion chamber to observe the fan
Ensure NO demands
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

The fan should run at high speed for approximately 30 seconds, it should then slow down and continue to run slowly.
Does the fan behave as described?