Your boiler's fan is spinning faster than it should, which is preventing the system from safely managing the air and gas flow needed for heating.

Fan speed too low: Fan harness and connections, Fan, Otherwise replace control board.

What causes this fault?

This fault occurs when the boiler's internal computer loses the ability to control how fast the fan is spinning, causing it to run at an unsafe maximum speed. The most common reason is a communication failure between the fan motor and the main control board, often caused by a worn-out component or a loose electrical connection. Because the boiler cannot accurately balance the air and gas mixture, it shuts down as a safety precaution.