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

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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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