Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove fan
Remove the burner

Observation:

Inspect heat exchanger flue ways for debris
and cleanliness.
Inspect burner for damage and good general condition.
NOTE:
This component requires the air/gas manifold and fan assembly to be removed. Therefore the gasket between the gas air manifold and the heat exchanger must be changed.