Swapped OEM catalytic converter for aftermarket one; now it sounds horrific

i would get under there and test for leaks. check all the joints and muffler

 

You very likely have a misalignment where the flange/donut mount to the exhaust manifold, or you forgot to install the O2 sensor...

 

I used a $125 Amazon cat sounded fine and car ran better. No codes.

Hope you didn’t use the donut that came with the cat should have used the original donut. The donut that came with my cat was a joke.

And Don’t understand the came in 3 pieces.

Give us the model of the cat you bought. Linky.

 

I don't know what that fits but it won't fit a G2 Prius. That's not even close. Looks like its got an extra resonator on it too.


Here's the one I used it fit perfectly. Only small issue is the output end is bigger than the stock pipe so the welder used a common sleeve to mate to the stock exhaust pipe. Which is perfect as now the place where thieves would cut your now cutting through 3 pipes at once. Much more difficult and louder and longer to cut.

Cost me $100 to have it all welded up. $80 and a $20 duke. Ran super quiet and car actually ran better.
And no emission codes. The O2 bung fit perfect too.

The donut that comes with this amazon cat is wrong much smaller so I was thinking that probably my donut would be worn out like yours so I bought a new donut also at the dealer online before I went to the welder. If my donut was ok I would return the new one. Glad I did as my donut was a mess too. Think new one was $35.
I included pic of the part number for the new donut.

I bought 2 new header bolts too but they were ok.

It is imperative you use the correct donut unless you like leaks and noise and smell of exhaust in the cabin.

The amazon cat is now $79 I see. I paid $125 for it 2 years ago.

I sold my OEM cat on ebay for I think $1500 been 2 years now can't remember exactly.

I painted my new Amazon cat bright orange with header paint too. See pic:

 

Looks like a Gen 2 exhaust to me.....

 

Put a rag over the exhaust tip at the rear of the car while it is running. Have a second person identify where leak(s) are located.