Your boiler is briefly checking its fan and clearing the system before it starts heating your hot water.

What causes this fault?

This is a normal part of the boiler's startup process where the internal fan spins up to clear the combustion chamber before the burner ignites for hot water. It isn't a fault, but rather a brief preparation phase to ensure the system is safe and ready to heat your water efficiently. You will usually see this appear for just a few seconds before the boiler begins its normal operation.