Has anyone replaced the cats on a v12 5.3 in california?

There are 4 of them. ...currently what is on mine is not a ca compliant cat. the pre cats are original... Would I have to replace the precats or can I leave them and just replace the two rear since they are aftermarket with no ca number on them?

the cats I find for ca are:

Magnaflow front?? rear?? 337303 (1.75") 337304 (2") , 337305 --$130
Walker 80256 -rear --$85
Walker 80401 -front --$85

Also, currently all four cats are in the front welded onto each other. Is the stock cat suppose to be in the middle of the car and one pre-cat up front?

I can replace all four for $320 + $400 labor. or replace two for $160 +$200 labor.

 

What are the laws on using aftermarket cats? Can I put on larger cats made for a truck? Do the numbers on the cat need to match with the car?

I have two pre cats for my 04 xjs 4.0 lying around, which I never used. They are spec'd for large trucks and vans where the ones spec'd for the v12 are smaller (less weight). The diameter on these cats are correct fit for the 5.3 (actually the wrong fit for the 4.0 , which is why I never used them) and they are listed as for a 4.0. Will they see it does not match my car exactly and fail me?

 

I did all 4 (92 V12, CA car)

We discussed the options your are considering, and the conclusion was the 2 front cats would probably be enough to get it to pass smog by the numbers, but a picky tester may fail it for either not having the back cats, or having non CARB approved back cats.

"Aftermarket" vs OEM..... my understanding is as long as it has a CARB sticker/mark on it, they don't care if it;s stock or aftermarket. I think the cats also have to be in the same place as the originals, and the same quantity

I aced the SMOG check, so I guess thats good.