Even though I have had two cats replaced on different cars, I had no idea how to diagnose it beyond the code. And even if one has the code, it is still a good idea to confirm.
I recently ran into a CarWizard video with a relatively simple way to diagnose. And lo and behold, I searched if Scotty had one and he does! Scotty has a diagnosis video that is 15 years old!
Both different methods, and both useful, depending on the tools you have. Here they are:
CarWizard: https://youtu.be/SwWcPPjoRwY?feature=shared&t=4200s
Scotty: https://youtu.be/IzyvL5tQLzU?feature=shared
Yep and since then I have even higher tech methods. But they require a $2,500 computer program and the hardware to plug into a car. That's what I use is mechanic. I just took it up. Do what the computer tells me to do and then it will tell what percentage the catalytic converters are worn out
but what's it called??