Your boiler is simply showing its current pump setting status rather than a fault; it indicates whether the pump is running constantly or only when needed.

Pump mode (Eco/Comfort)

What causes this fault?

This is not a mechanical failure but rather a status indicator regarding how your internal circulation pump is programmed to behave. It appears when the boiler is set to either run the pump continuously for constant heat circulation or only activate it during a specific heating demand. There is no physical component at fault; the boiler is simply communicating its current operating logic.