Your boiler is simply running a routine internal test to check it can operate at its maximum power level after starting up.

Test program where the appliance is operated in full load after successful ignition

What causes this fault?

This is not actually a fault, but a built-in safety and performance check. It occurs because the boiler needs to confirm it can safely reach its highest firing rate immediately after the burners have lit. Once the system satisfies itself that everything is venting and heating correctly at full power, the code will disappear and normal operation will resume.