2001 Chevrolet Cavalier OBD2 Code P0420 Bank 1 4-Cylinder: How to Fix Cat Converter Failure at 120,000 Miles
Hi everyone, I own a 2001 Chevrolet Cavalier with a 2.2L 4-cylinder engine. For the past six months, this vehicle has been failing its NY smog inspection due to OBD2 diagnostic code P0420 — 'Catalyst System Below Threshold (Bank 1)'. I've already replaced both oxygen sensors and the original exhaust system is now at approximately 120,000 miles. There are no visible leaks in the exhaust system. The car previously had starting issues, which were resolved after replacing the fuel tank with a used one that includes a pump — now it starts reliably even without a full tank. The OBD2 scanner shows both oxygen sensors passing during checks, but the P0420 code returns after about a week of driving. I've also completed a full tune-up. Despite these efforts, the issue persists. My question is: How can I properly test the oxygen sensors? Does a 'pass' from an OBD2 scanner mean they're functioning correctly? Is it possible that the oxygen sensor was incorrectly installed — especially since I used a generic 4-wire unit and wasn't certain about wiring connections? What should I do: replace with OEM, aftermarket (like MagnaFlow or Eastern), or consider a cheaper 'mil eliminator' device? I've heard of Walker direct-fit converters being reliable for the price. Any advice would be greatly appreciated. Thanks! Jon
Given that your vehicle has 120,000 miles on the exhaust system, it's highly likely that the catalytic converter is worn out and needs replacement. Since both oxygen sensors have been replaced and the OBD2 scanner shows them passing, this suggests they are likely functional. However, be cautious when purchasing a catalytic converter — avoid 'fits-all' models unless you're prepared to replace it again soon. Always prioritize quality over price for long-term reliability.