Your boiler is displaying a service setting notification to run the pump at maximum speed, which usually indicates an installation or maintenance mode rather than a dangerous mechanical failure.

To set the minimum CH pump speed to 100%

What causes this fault?

This code appears when the boiler has been manually adjusted to run its internal pump at maximum speed, typically during the initial setup or a system cleanse. It essentially means the energy-saving variable speed feature has been overridden to help flush air or debris out of the system. While not a mechanical breakdown, the boiler will stay in this high-performance mode until the setting is changed back to automatic.