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Q: Check engine light is on. Code PO420. What could it be? on 2002 Pontiac Sunfire

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
The car runs fine, smooth. No missing or backfires.
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yes scan o2 readings and see if post 02 is reading correctly as compared to pre catt o2. also measure temp going into catt and out of catt there should be between 50 and 100 degrees diff between the 2
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