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Posts Tagged: P0420 after cat replacement

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, … Read More


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