Catalytic system question

My 4.7L Sport(125k miles) threw the dreaded CEL, VSC Trac On/Off lights. Code is P0430, catalytic system efficiency is below threshold(bank 2). Took it to a trusted mechanic(he did the DT headers after the manifolds cracked) and he said start with the O2 sensors and replaced all 4, clearing the codes. There's also an exhaust leak on the Y pipe, right on the Y. The lights came back on within a dozen miles. I've run 3 bottles of Cataclean through just for kicks. Reset the codes and the lights stayed off for 30 miles or so and came back on. Looks like I'm in for a CC replacement. Slid under it this weekend just to look around and am a little confused. I've been told, and read, that the 4.7L V8 has 4 converters. I feel like an idiot but I only see 1 on each side. They're welded to the headers, with a O2 sensor before and after. The converters then each couple to the Y pipe. The Y pipe is a single piece, and there's nothing between the couplers immediately behind the converters until the Y pipe is coupled to the exhaust pipe. Am I missing something? Every parts site lists 4, front and back on each side but I just don't see 4. On a different note, is it possible the exhaust leak, which is pretty bad, couple be causing the CEL? If I can get a decent pic, I'll post what I'm seeing.

 

There’s usually two somewhere underneath the vehicle which are the ones you’re probably seeing, then there is the other two up in the engine bay that usually are proceeding the exhaust manifolds.

 

Yes, an exhaust leak can cause sensor misreadings. Fix that first.

Does your mechanic use OEM parts? If not, chances of returning failures are very high.

The TOYOTA cat is very high quality. It seems to be rare that it's the problem.

Also, I've seen people put on O2 sensors when they need AF sensors. They look the same but are not. Only the dealer can tell you based on your vin which one you need.

 

Good to know on the AF vs O2 sensors. Will see if I can get an answer from a dealer on that. He did use OEM sensors at least. Going to take care of the leak next. Still haven't figured out why there's only 2 cats. It goes headers, welded to the OEM cats, then nothing but pipe from them that meet at the Y. It's actually a single piece, just a big Y really. I guess whoever had it before me could have cut the others out. The leak is on the Y anyway, so the entire piece will get replaced. Thank you both for your replies.

 

The cats with S01 before and S02 after are part of the exhaust manifolds, which are notorious for leaks right at the flange (oft repeated there are two kinds of Toy 4.7L exhaust manifolds, those with leaks and those that will leak soon) and the Toyota ones are a bit pricey thanks to the cats built in. Any aftermarket ones will be lucky to go a year before it starts throwing codes.
Sounds like previous owner to you removed the cats on the single after the Y, or some lowlifes might have removed them for him.

 

There's two cats up front attached to each manifold with a pre and post 02 sensor. Then shortly after are the secondary cats with no 02's right before the Y. On a stock setup anyway. My cats lasted till 200k and bit the dust. i did try to replace the upstream 02 after they tested it, but it fix the issue.