I'm dealing with my mom'* 2013 Malibu....it had a P0420 Code..... I replaced the Catalytic Converter and put in new o2 Sensors....the CEL is still coming on....what else would cause a P0420 Code?

Have the codes been cleared since installing the new cat?

How far has it been driven since the new cat?

What brand cat?

No exhaust leaks?

A smoke test show no leaks.....and it was a cat we got from a local parts dealer. And, yes, the codes have been cleared.

Recheck the flex pipe after the cat. have seen a lot of those leak setting the p0420. If the post cat O2 is ranging like the pre cat O2 you either have a leak or bad cat. Hope this is helpful.

If there was a leak in that would not a smoke test have detected that? I did the smoke test from the tail pipe and I saw no leaking anywhere.

What brand cat?

We are in Toronto so the manufacture that was given us was called, "First Choice".

Looks like an Ebay special. No real web site. In a tiny space in some little run-down business park. Check this out: https://g.co/kgs/SE3vzWr

Check their Ebay reviews. Not the positive ones (although some of them are interesting reading), but the neutral and negative ones: https://www.ebay.com/str/firstchoice...?_tab=feedback

Do some of those sound familiar?

I'd return this part right away and get something from a reputable manufacturer . . . except there probably is expensive shipping to return it (according to many of the reviews there) so maybe better to cut your losses and just get a good one instead.

. . . and if your "First Choice" manufacturer is a different [first choice] than "FirstChoiceExhausts" then they should probably change their name to avoid being confused with whatever is going on there.

First Choice would be my last choice. They sell junk.