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Q: code reads p0421 on 2000 Saturn LS2

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
code p0421 on computer. car runs perfect. what possible causes?
No car image 94a1663db56199c5353592009e34aaa51078a2469bed068bb8d6f0ba43accf97
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Common causes are: Defective Warm Up Catalytic Converter, Engine misfires have damaged the Catalytic Converter.
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