If you car was originally from California, it probably had a specific cat that meet those emissions. If you live in Nevada, and are not going to go back, I would not bother replacing it with a California compliant one, as they are quite a bit more expensive. I would just go to a local to you muffler shop and have them install a generic one. They will be able to modify your exhaust to make something new fit properly. If you really want to try it yourself, I recommend looking through the ones on rockauto and finding the one that matches what you had
Unfortunately, you learned the hard way that the people at Autozone are not mechanics. All they do is read the codes and tell you what might be the most likely cause. Honestly, I have talked to quite a few people who work there that seem to know almost nothing about vehicles.
2013 CHEVROLET IMPALA 3.6L V6 Catalytic Converter | RockAuto
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Unfortunately, you learned the hard way that the people at Autozone are not mechanics. All they do is read the codes and tell you what might be the most likely cause. Honestly, I have talked to quite a few people who work there that seem to know almost nothing about vehicles.