2010 Chevy Impala 3.5L

A P0420 doesn't always mean a to replace the cat. Were you having other issues lately like misfires and poor running? A P0420 code can also be caused by a faulty oxygen sensor, exhaust system leak or an intake leak. Have a decent mechanic look the car over before replacing your cats, it might save you some money.

Once the other possibilities are ruled out as mentioned above you must replace the catalytic converters with converters that are CARB (California Air Resources Board) compliant. Otherwise your check engine light will come right back on. A Federal Emissions converter will not work on your vehicle! Find out how many downstream oxygen sensors you have. If you have one behind each converter you'll need to find out which one is throwing the P420. Then replace that converter. A live data scan will tell you. If you only have one downstream oxygen sensor behind the second converter replace that converter only with a CARB compliant one. The downstream oxygen sensor is only there to monitor the catalytic converter, it does nothing else. The computer will test the catalytic converter as you're driving by momentarily enriching the air to fuel mixture to create excess emissions which the converter should "eat up" before the exhaust leaves the converter. If the computer sees a spike in the signal from the downstream oxygen sensor it knows the converter is not working properly and the P0420 is set and the check engine light comes on. The computer should see a flatline signal from the downstream oxygen sensor. Even when the catalytic converter test is run. Other than a bad catalytic converter, faulty wiring or an exhaust leak can trigger a P0420. Hope that helps! Jim