Your boiler is simply preparing its internal pump to circulate water before it starts heating up your hot water supply.

DHWmode:Pumppre-run

What causes this fault?

This fault occurs when the boiler's internal sensors detect that water isn't moving quickly enough through the system to safely transfer heat to your hot water. The most common reason is a blockage caused by limescale or debris in the heat exchanger, or a mechanical failure in the secondary pump that pushes the water around.