Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Remove the combustion control unit and gain
access to the heat exchanger flueways

Measurement:

Visually inspect the following (inspection mirror may help).
  •  Heat exchanger flueways are clear and unrestricted.
  •  Fan bearing plate, ensure opens fully and closes without sticking,
    this can be achieved by placing the fan assembly upside down
    (bearing plate open) and correct way up (bearing plate closed).
    Note:
    If the visual inspection of the above is satisfactory,
    do not reassembly the appliance.