Katalysatorträger gebrochen oder zusammengefallen

The ceramic honeycomb inside a catalytic converter cracked, broken up or collapsed. The converter then fails in two ways at once — it no longer converts, because the surface area has gone, and it restricts the exhaust, because the debris obstructs flow.

The restriction is the more damaging half and produces the more confusing codes. Backpressure starves the engine, so airflow, boost and pressure sensor codes appear while the actual fault is downstream of all of them. A rattle from the exhaust when tapped, or a marble-like noise on start-up, is the confirming evidence and is free to check.

Substrates break from thermal shock or impact rather than age. Driving through deep water onto a hot converter, a heavy speed bump, or the sudden heat of a misfire dumping fuel into it. Look upstream for what broke it before fitting a replacement, and check that no debris has migrated further down the exhaust — a fragment lodged in a rear silencer produces a restriction that survives the new converter.