Rick's Free Auto Repair Advice

P0420 after Catalytic Converter Replacement

Why do I get P0420 after Catalytic Converter Replacement

Trouble Code P0420 is “Catalyst System Efficiency Below Threshold.” This coded sets if the computer see similar readings from both the pre- and post- converter oxy
gen sensors. The pre-cat sensor should switch rich/lean. But the post-cat sensor should give a fairly steady reading, indicating that the converter is doing it’s job.

Independent repair shops report that the majority of these P0420 codes after converter replacement are due to the installation of AFTERMARKET converters. They’re less expensive than original equipment, well, except that they DON’T WORK. You heard that right. When the aftermarket cats are replaced with OE, all the trouble codes disappear. If you’ve had your cat replaced with an aftermarket converter and are getting a cat related trouble code, take it back to the shop and pay the difference for an OE converter.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat


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