Your boiler has stopped working because it cannot detect a flame, meaning it has safely shut down to prevent gas from flowing without being burned.

The description indicates this is displayed when there is a failure in the flame safety device, specifically if the main burner is extinguished or the gas inlet cock is turned off.

What causes this fault?

This fault occurs when the boiler's control system fails to confirm that a flame has been successfully lit after a sequence of attempts. The most common reasons include a lack of gas reaching the burner, worn-out sparking electrodes that fail to ignite the fuel, or a dirty sensor that can no longer 'see' the flame even if it is actually present.