Your boiler has stopped working because the internal fan, which safely exhausts fumes, is not spinning correctly or communicating with the system.

The fan is stationary or there is no fan speed signal.

What causes this fault?

This fault occurs when the boiler's control system detects that the fan is either not spinning at all or is spinning at the wrong speed to safely expel exhaust gases. The most common reason is a mechanical failure within the fan motor itself, though a loose electrical connection or a fault on the main control board can also stop the fan from receiving power.