Your boiler has stopped working because the internal fan isn't spinning fast enough to safely start the heating process.

Burner in fault position. Fan speed too low during burner start. Check the fan, the fan cables and the power supply to the fan.

What causes this fault?

This fault occurs when the boiler's computer tries to start the heating process but detects that the internal fan isn't spinning at the required speed to clear out exhaust gases. The most common reason is a mechanical failure within the fan motor or a communication issue where the control board can no longer send power or instructions to the fan.