Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
CH Demand
Operating Switch in Heating & Water position
Remove combustion chamber cover
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Is the Fan running?
Note:
Do not take Noise or Vibration as indication that the Fan is running.