Your boiler is simply reminding you that its annual service is due soon; it is a notification rather than a breakdown.

Service timer function

What causes this fault?

This notification is triggered by an internal clock within the boiler’s control board that tracks the time elapsed since its last professional maintenance check. It is not caused by a mechanical failure or a parts breakdown, but rather a pre-set countdown reaching its limit. Once the timer completes, the display shows this code to prompt you to book a Gas Safe engineer.