Your boiler has detected a signal suggesting a flame is present even when it should be turned off, so it has shut itself down as a precaution to ensure your safety.

Safety switch-off: Flame simulation

What causes this fault?

This fault occurs when the boiler's internal sensors detect a flame after the burner should have already shut off, or before it has even started. The most common cause is a moisture buildup or carbon deposits on the sensing electrode, which creates a false electrical signal that tricks the control board into thinking a fire is present.