Sekundärluftpumpe ausgefallen

The pump that blows fresh air into the exhaust ports for the first minute or two after a cold start, so unburnt fuel keeps burning in the manifold and brings the catalyst up to temperature faster. The system runs only briefly and only when cold, which is the fact that makes it hard to test: by the time the vehicle is on a ramp with a scan tool connected, the pump is no longer supposed to run.

This entry is the pump failing electrically or losing output, as opposed to one that has mechanically seized — the two are separated in this corpus because they present differently. A failing pump still turns but moves too little air, so the system stores an insufficient-flow or incorrect-flow code while the pump is plainly audible. A pump you can hear is not a pump that is working, and that is the trap.

Water is the usual killer. The pump sits low, its intake draws from a wheel arch or the engine bay, and condensation from the exhaust side can find its way back through a failed check valve into the pump housing. So a failed pump argues for inspecting the check valve in the same visit — a new pump behind a failed valve is a repair with a known expiry date.