Your boiler is simply preparing its internal pump before starting the heating or hot water process.

pump pre-run

What causes this fault?

This isn't actually a fault, but rather a brief status message where the boiler circulates water internally before firing the burner. It ensures that any trapped heat is cleared and proper flow is established to protect the heat exchanger from overheating. Once this short preparation phase is complete, your heating or hot water will begin as normal.