I’m happy to see the answers as well as the questions. OP stuck with it for this long.
Success is always good.
A cat can always fail even with no reason. The google stuff didn’t give reasons for that. I got the impression that it was the P 0420 code that was discussed there.
I can only say maybe when we talk about the resonators. Thanks for keeping the info going.
Thanks for all the replies everyone. I took off the other stock resonator and put 2 glasspacks after the cat and before the mufflers to use as resonators. I datalogged my car and got a retune from MPT. My car seems to be running a lot better. The only thing that puzzles me is why my catalytic converter was physically damaged and broken pieces inside when I never hit the bottom of the car on anything. Weird.
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The cat’s guts consists of a solid ceramic matrix. Any cook will tell you that ceramic stuff can break for all sort of reason, thermal stress, hard knocks, etc. Just going over a bump that causes the suspension to bottom out could crack it, and over time thermal stresses will eventually break it apart. And of course there’s always the possibly it was defective from the manufacturer. Good for you for getting your Mustang back on the road.
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I’ve had them come apart as well with no misfires or abuse.
Hi, it clogged cat, if you put the cheap the code can be back after 500 km, you could have bad spark or bad wire this cause cat fail.
Somebody said you could put paint thiner to the gas and try to burn out the cat.,google for more info.
Then let “somebody” ruin their own car by doing that. It doesn’t work and it is not good for the engine.
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Check Scotty Kilmer’s youtube.
this site has no good emoji for “roll my eyes” …
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No, thank you. I’ll take a hard pass! 
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People put other stuff,so you don’t know what are you put in???
My 2004 Toyota Tacoma 2.7L has two cats (one pre-cat) and one (post cat) I had another code but I needed to replace my cat The Muffler shop said, it was the second one and replaced it that cost me about $700 took a day to get and put in. Took months to clear the code finally passed smog but only after extending my registration for $50.00. I was told by the shop that those are suppose to last for the life of the car. but my Toyota Tacoma had over 225K miles. I am told it the lining of the wire grid the platinum coating will evenly wear away.
That’s very annoying for the DMV to extract another $50 from a car owner just b/c the car’s computer takes a while to realize the new cat is doing its job. It’s not the car owner’s fault after all the computer takes so long to figure out what should be obvious. I expect they DMV will keep doing it though, b/c it generates income for the state.
this comes close:
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