Your boiler is performing a routine automatic cleaning and filling cycle of its internal drainage system, which is a normal process and not actually a fault.

Syphon-fill programme

What causes this fault?

This code appears because the boiler is automatically filling its internal 'trap' with water to ensure harmful flue gases cannot escape into your home. It typically occurs after the boiler has been powered off and back on, or if the unit has been sitting idle for a long period. The system is simply performing a safety check and preparation cycle before the burner ignites.