Smeg oven shows Er11 and the cooling fan runs continuously or not at all.

Safety first: Disconnect power before accessing internal components.

• Observe fan behavior: If the fan never runs, check for overheating and error codes. If it runs constantly even when the oven is cold and off, suspect a stuck relay or triac.
• Test the fan motor: Access the tangential cooling fan at the top or rear of the oven. Check that the blades spin freely. Measure resistance across the motor; an open circuit indicates a failed motor.
• Check supply to fan: If you are qualified, restore power and measure voltage at the fan terminals when the oven is on and off. Voltage present at all times suggests a stuck relay/triac on the main control board.
• Replace faulty parts: Replace the cooling fan motor if it is open-circuit or seized. If the motor is good but control is incorrect, replace or repair the main control board.