Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove fan assembly from appliance

Observation:

Undo the three screws securing the mixing chamber housing.
Check the fan membrane is not sticking or worn.
Inspect the fan scroll for dust
build up and check it spins freely.
Note:
As fan assembly has been removed per
manufacturer instructions the gasket between
the gas air manifold and the heat exchanger must be changed.