Your boiler is flagging a minor technical adjustment error regarding its internal fan speed settings.

Offset for maximum speed

What causes this fault?

This fault occurs when the boiler's control board detects that the internal fan is operating at a speed that doesn't match the expected safety settings. It is usually caused by an electronic communication error between the main circuit board and the fan, or a slight decalibration in how the gas valve is being controlled. While the boiler may still operate, it signifies that the internal components are not perfectly synced.