Your boiler has failed to start properly and the fan is running constantly to clear out the system, preventing the heater from firing up.

Burner in fault state. Fan stop not achieved. Check the fan, check the fan cables, check the fan control.

What causes this fault?

This fault occurs when the boiler's internal computer detects that the fan is spinning at the wrong speed or failing to stop when it should. Because the fan is responsible for clearing out dangerous exhaust gases, the system enters a safety mode and prevents the burner from lighting. The most common reason is a communication error between the fan motor and the main control board, often caused by worn-out electrical components.