Your boiler is simply going through its normal startup sequence by circulating water before it attempts to ignite.

Pump running

What causes this fault?

This code appears when the boiler begins a new heating cycle and starts the internal pump to move water through the system. It is a natural part of the startup process designed to ensure there is enough water flow before the burner lights. You will usually see this for a few seconds before the boiler settles into its normal heating mode.