Catalytic converter

In order to reason objectively, the detected error code is necessary.

A constantly lit check engine light is dangerous because you get used to it and can miss a serious problem.

 

You can't damage the valves, if the car is running rich. You can fry the CAT if it's running rich or you have excessive oil burn. We can't see, smell, or hear the car over the internet, that's why we're asking for error codes.

There's a communications issue here. Either you misunderstood what the mechanic told you or the mechanic is full of CRAP. As I explained above. There's several ways to kill a new CAT or that CAT was defective. That's why we ask for the error codes. A good, professional repair shop would've checked for those other issues, before installing a new CAT; because they need to warranty their work. If a customer drags in their own part and tells them to just replace this. That's all they're going to do. The only warranty your going to get is that it won't fall out of the car.

@MAX2 stated; if another fatal issue crops up; you won't know, until the car just stops running - since your CEL is already lit.

 

Those P-codes are what we need to do a remote diagnostic. Anyone can say what they think the problem is (ie. low cat efficiency - P0420); but that's their interpretation of that code. Was there any other codes the car kicked out? A lot of us with experience fixing cars, can tell you the most likely part failures, when you give us a bunch of error codes the ECU kicked out. But when you tell us your mechanic thinks is this - what do you want us to say?

Price has nothing to do with it. Both Amazon and E-bay are flooded with counterfeited parts. If your car is running correctly, it shouldn't have destroyed the CAT. Figure out what the warranty is on that part, normally 2 years or 24K miles - but you'll need to prove that it was installed correctly and the car was also running correctly. If you had a repair shop sell you the entire job, they would be on the hook for the warranty repair.
Catalytic Converter Warranty: Owners' Peace of Mind

Good Luck.....