Post O2 setting code P0420?

2006 CHEVROLET UPLANDER
103,000 MILES • 3.9L • 6 CYL • 2WD • AUTOMATIC
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FZ566QA
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P0420 code sets. Replaced the Cat because of the milage at a local muffler shop and the code continues. Replaced the Post O2 with one from AutoZone. Code still sets. If the a Heated O2 Sensor Relearn that must be performed with a tool? Do I need to order a Bosch/Denso O2 (Walker Brand from Auto Zone)?
Jan 6, 2025 at 10:56 AM
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This can happen if the exhaust system is leaking and letting air into the system, but to me it sounds like the cat is bad still, did they replace it with a new cat or rebuilt? Did you see your old part? Also, I would check the fuel trims to see if they are both rich, which the reading will be -10% and above. This means the system is running rich.

This guide can help:

https://www.2carpros.com/articles/why-an-automotive-engine-will-run-rich

I don't think it will be the oxygen sensors. Please go over this guide and get back to us.
Jan 6, 2025 at 11:54 AM