Safety first: Disconnect the appliance from mains and ensure the oven is cool.

Inspect fan blade: Remove the oven rear inner panel to access the convection fan. Check for food debris or screws obstructing the blade. Remove obstructions.
Test fan motor: Spin the fan by hand; it should rotate freely. If stiff or seized, the convection fan motor must be replaced.
Electrical test: From the rear of the cooker, access the fan motor terminals. Measure resistance; an open circuit indicates a failed motor.
Check wiring and connectors: Inspect for heat damage or loose connections between the fan and control board. Repair or replace as needed.
Control relay: If the motor is good but never receives voltage when fan mode is selected, the fan relay on the electronic control board may be faulty and the board should be replaced.
Re-test: After repairs, run the oven in fan mode to confirm the fan runs smoothly and E21 is cleared.

Electric oven shows E21 and the convection (fan) function does not operate correctly.

Possible Causes