Rückschlagventil der Sekundärluft festgefressen

The one-way valve between the secondary air pump and the exhaust, seized so that air cannot pass. Its job is to protect the pump from exhaust gas, and its failure mode is doubly damaging: seized shut it blocks the system, and leaking open it lets hot, wet exhaust back into the pump and destroys that too.

Water is the mechanism behind both. Exhaust gas carries a great deal of water vapour, which condenses in the cool valve when the system shuts down, then corrodes it and freezes in it. So a failed check valve and a failed pump usually arrive together, and replacing one without the other is a repair with a short life.

Distinguish it from a blocked passage in the head, which produces the same insufficient-flow code from a different place. Remove the valve and test it by hand — it should pass air one way and seal the other, cleanly. A valve that passes both ways, or neither, is the fault; one that works freely points the diagnosis downstream into the exhaust passages, which is the more expensive answer and worth arriving at rather than assuming.