Your boiler has detected that its internal fan is spinning at the wrong speed, which means it cannot safely exhaust fumes and has stopped working as a result.

Fan speed too high.

What causes this fault?

This fault occurs when the boiler's control system loses the ability to regulate the fan speed effectively, often due to a mechanical failure or an electrical communication error. The most common reason is a failing fan motor or a fault within the main circuit board that prevents it from sending the correct signals to manage airflow.