Your boiler has detected that a flame is still burning inside even though it has been told to turn off, which means it has safely shut itself down to prevent a hazard.

Flame detection fault (flame presence for 5 seconds after burner stopped)

What causes this fault?

This fault occurs when the boiler's sensors detect a flame signal at a time when there shouldn't be one, such as during the initial startup checks. The most common reason is a faulty flame sensing electrode that is sending a false signal, or a carbon buildup that is allowing electricity to leak across the sensor. In some cases, the main control board may be malfunctioning and incorrectly interpreting signals from the burner.