348 Meow (high flow cat issue)

Your mixture is off and you killed your cats. You need to fix your mixture control getting right A/F ratios under all closed loop driving and then see if the cats will burn off all the crap or if you killed them and now need new cats. Cats are rarely the problem. Cats only respond to conditions presented. Cats don't fail because they are bad. Cats fail because something is wrong upstream killing them. Dead cat is a symptom where you need to find the cause.

 

When these cars were newer at major service did things like techron for fuel injectors. As the cars aged found need more care so some of use did full FI blueprinting to ensure car fuels deliver as designed. Then we always had electrical issues and finally with re-pin of connectors we got good electronics to deliver the right signals. So you have to either find out where you have faults or make sure you have a system capable of not producing any faults.

 

I have Nouvalari cats on my 355. Had them for 11 years. They have been fine. But, like you I had an ignition problem (bad coil) on bank 1. Excess fuel in the exhaust caused the cat to over heat and the rear most section of the core on the 1-4 side was damaged. I had to remove the cat and clean out the damaged section. After that the cat still worked good enough to pass emissions.

These high flow 200 cell metal cores are very easy to damage from over heating due to excess fuel. I suspect you have a similar problem but perhaps worse. You need to pull the cat and look at the core.