Your boiler has detected that you have turned on a hot water tap and is currently working to heat water for you.

What causes this fault?

This code appears when the boiler's internal flow sensor detects water moving through the pipes after you open a hot water tap. It physically indicates that the internal diverter valve has shifted position to prioritise heating your water instead of your radiators. This is a normal functional message and does not indicate a mechanical failure or a breakdown.