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A converter whose substrate has broken up or partially melted, restricting the exhaust so the engine cannot breathe out. The converter has stopped being a catalyst and become a plug, and the codes it produces name airflow, manifold pressure and boost rather than emissions — which is why it is so often missed.

Backpressure starves the engine in a way that mimics an intake fault. Airflow readings fall short, manifold pressure behaves oddly, the turbocharger cannot achieve boost against the restriction, and the complaint is a severe power loss that worsens with revs. A vehicle that pulls acceptably to two thousand revs and then hits a wall is describing a restriction, not a fuelling problem.

Measure backpressure directly rather than inferring it. A gauge in the upstream oxygen sensor port settles it in minutes, and the reading should stay low as revs rise. Ask why it failed while it is off: a substrate does not break up on its own, so a prolonged misfire, over-fuelling, or an impact is behind it — and fitting a new converter without finding that repeats the failure.