Your boiler is currently running a background self-test to check it can operate safely at its lowest power setting.

Test program where the appliance is operated with minimum gas volume (ignition gas volume) after successful ignition

What causes this fault?

This code appears when the boiler automatically performs a internal calibration test to ensure it can sustain a stable flame at the lowest possible gas level. It is a normal part of the boiler's software routine and happens automatically after the burner ignites to verify efficiency and safety. You do not need to take any action as the boiler is simply checking its own homework before continuing to heat your home.