Catalytic Converter Diagnosis

A catalytic converter can fail in two ways — it can fail chemically where the catalyst material is no longer converting emissions effectively, or it can fail physically where the internal substrate breaks apart and creates a restriction or rattles inside the housing.

These are catalyst efficiency below threshold codes. The PCM compares the upstream and downstream oxygen sensor signals. If the downstream sensor starts mimicking the upstream sensor — switching rapidly instead of staying relatively flat — the converter is not processing the exhaust gases effectively. Before replacing the converter, always verify that the engine is running correctly first. Misfires, rich conditions, coolant leaks into combustion, and oil burning can all poison a converter. A new converter installed on an engine with an unresolved running problem will fail again.

A converter with a melted or collapsed substrate restricts exhaust flow. The engine loses power progressively as RPM increases because the exhaust cannot exit fast enough. A vacuum gauge at idle reads normal but drops steadily as RPM increases and stays low — this is the classic restricted exhaust pattern. You can also check exhaust backpressure directly with a pressure gauge in the upstream oxygen sensor bung. Specification varies but generally anything above 1.5 to 3 PSI at 2500 RPM indicates a restriction.