Catalytic converter failure: symptoms, causes, and why the repair costs $1,500+
Understanding why catalytic converters fail, the P0420/P0430 codes, and how to tell if you need a new cat or just an oxygen sensor.
The catalytic converter converts toxic exhaust gases (carbon monoxide, hydrocarbons, nitrogen oxides) into less harmful emissions using precious metals like platinum, palladium, and rhodium. Those metals are why a replacement costs $1,000-2,500 for the part alone — and why catalytic converter theft has become an epidemic.
The most common trouble code is P0420 (catalyst system efficiency below threshold, bank 1) or P0430 (bank 2). This code means the downstream oxygen sensor sees too much variation, indicating the catalyst isn't cleaning the exhaust effectively. However, P0420 can also be triggered by a failing downstream O2 sensor ($150-300 to replace) rather than the converter itself. A good technician will test the sensor response rate before condemning the converter.
Symptoms of a failing converter include reduced power (especially uphill), a sulfur or rotten-egg smell from the exhaust, and a rattling noise from under the car (the internal substrate has broken apart). A completely plugged converter will cause the engine to stall or refuse to rev above 2,000-3,000 RPM.
What kills catalytic converters: engine misfires (unburned fuel enters the converter and overheats it), coolant leaks into the combustion chamber (contaminates the catalyst surface), excessive oil burning, and running rich for extended periods. Fixing the upstream cause is essential — installing a new converter without addressing the root cause will just kill the replacement too.
Information is for educational purposes only. Always consult a qualified mechanic for diagnosis and repair. AutoAt's knowledge base is AI-assisted and regularly updated but may not reflect the most current manufacturer specifications. Always verify critical specifications with official service manuals. Repair procedures described here may require professional tools, training, and certifications. Attempting repairs beyond your skill level can be dangerous.