PS- I will get some more pix up soon of my monte the ones I have up do not do it justice I have done many more things to it since those pix.

 

I own a 2002 MC SS, N/A L36(very modded) and have driven with a 3" in/2.5" out, Random Tech(R/T) downpipe for 3.5 years and stock cat-back. The first 2 years was with a stock PCM. Never threw a code. Now if you use a version without a cat(off road), then yes, you will throw a code. If you simply disconnect either O2 sensor and drive that way, you will throw a code. And, maybe, a 3" inlet/outlet might also throw a code, but that might depend on the cat type and still be rare. A 160-degree T-STAT will also throw a code. Even just a CAI, ST1 TB, headers, too-cold plugs and a 180-degree T-STAT, has thrown codes. It’s usually your combo of mods, not simply the downpipe.

 

to the original poster, you should look in the classified here on this site. i have a really cheap setup on the exhuast side for you! people can vouch for me. ceramic coated plog to keep engine temps cool and allow more free flowing exhaust....great for N/A 3800's. ceramic coated crossover pipe to keep your IAT's down, and a 2.5" bolt in downpipe w/ high flow magnaflow cat.....all ordered from zzp about a year ago all in great shape. $400 for everything + ship, or $300 + shipping for everything if you give me your stock downpipe and cat. the only reason for the stock dp is because i have smog in california and need it just for smogging. kind of a waste, only need it for a day but my loss is ur gain.
since im supercharged, i've decided to go with headers instead of this setup. it'll be better for me.
these parts new are worth about $600.

let me know. heres the link to my thread:

https://montecarloforum.com/forum/sh...ad.php?t=17230

 

p.s. the main code you'll see is the P0420 code, with means cat is at its threshold. if you purchase a pcm you can ask for it to be tuned out, meaning it will never throw that code after that. i had that done to my pcm before i even installed anything. pretty simple.