Your boiler has detected that the internal fan responsible for safely removing exhaust gases is not spinning at the correct speed, causing the system to stop for safety reasons.

Fan speed fault. Check flue and ventilation. Check air inlet manifold and pipe. Check for fan damage/fault. Check fan wire connections.

What causes this fault?

This fault occurs when the boiler's control system loses the ability to manage or monitor the speed of the internal fan. The most common reason is a mechanical failure within the fan motor or a communication error on the circuit board, meaning the boiler cannot safely expel exhaust gases. As a result, the system shuts down as a vital safety precaution to prevent dangerous fumes from building up.