Yeah the cat can fuck it up, My car originally when i bought it, With the maf plugged in, I couldn't rev beyond 4k. UNLESS the maf was unplugged. And i tried fixing everything that would cause my car to do that, and idle awkwardly, Never even thought about the cat. Now my brand new downpipe gasket is blown and my exhaust mani is slightly cracked. If i even decide to fix it, I'm sazawing the cat off and replacing the mani. I've already got the muffler sazawed off so why not lol