Your boiler is simply letting you know that it is currently busy heating up the water for your taps or shower.

Boiler in hot water mode

What causes this fault?

This code appears when the boiler detects a demand for hot water from a tap or shower, causing it to divert its energy away from the radiators. It is a normal internal message indicating that the burner is firing to heat the water for your domestic use. Once you turn the tap off, the boiler will return to its previous state or go into standby.