Your boiler has paused its operation because it is not receiving the correct feedback signal from the fan that pushes exhaust gases out of the system.

Fan return flow

What causes this fault?

This fault occurs when the boiler's control board sends a command to the fan but does not receive the expected signal back to confirm it is spinning at the right speed. It is often caused by a mechanical failure within the fan motor or a communication breakdown in the electrical wiring between the fan and the main computer. Without this confirmation, the boiler stops for safety to ensure harmful exhaust gases are being correctly removed.