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

For service timer function refer to separate instructions.

What causes this fault?

This message is triggered by an internal clock within the boiler's control board that counts down the months since your last professional inspection. It is not caused by a mechanical failure or a broken part, but rather a pre-set software timer designed to encourage regular maintenance. Once the set time period has elapsed, the display shows this code to remind you to book a Gas Safe engineer.