[big_ol_litres]

I’m new so please excuse my lack of knowledge. I recently purchased an e60 550i and I am getting codes for a clogged bank 1 cat. I took off the exhaust from the cat back so that I could check the flow and temperatures on the first set of cats, and the flow seemed fine without having proper tools and am still getting codes in ISTA for a clogged/bad bank 1 cat. I have replaced all four o2 sensors.

My question is: can I drill out the mesh material inside my first set of cats to alleviate this if the new straight pipe doesn’t solve it? My current thought process is that the code is actually stemming from the back pressure from a clogged resonator or one of my second set of cats. I am designing and building a custom exhaust system that’s Y piped from the first set of cats to a 3” pipe to a Borla resonator and another Y pipe to a dual 3.5” exhaust tip.

Am I off on my reasoning? Apologies if this has been well covered elsewhere.