Your boiler is simply reminding you that its scheduled annual service is now due to ensure it continues running safely and efficiently.

What causes this fault?

This code appears because your heating engineer set a specific time interval or operation limit during the last visit to ensure the appliance is maintained correctly. It is not a mechanical failure, but rather a built-in timer that has reached its end date to help you stay compliant with your manufacturer's warranty. Once the engineer completes the annual safety checks and cleaning, they will reset this timer to clear the message.