First off, I have a 3.8, not a turbo (gonna leave that in there, I kept seeing all the CEL posts were in the 2.0 section, lol). I purchased, back in September of 2010, ARK downpipes, a GRIP V1 catback exhaust and an Injen CAI. I was told by ARK that I may or may not begin to get check engine lights or cels, which would be caused by the downpipes.
The downpipes, just like the stock pipes, have a short, round metal "tube" protruding from them that holds the O2 sensors in them. I was told that since the ARK downpipes are larger than the stock pipes, the amount of air and the O2 mixture would be different and that is what would cause the Cel's; but to "fix" it, they had some things called "CEL FIXERS" you could order online. They had initially said they would get them for me if I had the problem, which I did, intermittently, but I got tired of waiting for them to keep their promise and ordered them myself.
I received further verification when I would take my car to the dealer with the check engine light on. They would look into it and tell me that was what the problem was, the O2 sensors, and they too said it was probably because of the downpipes.
I just recently got the CEL fixers installed (after having them for about a year sitting in my glove box, lol) and they have substantially reduced the number of times the light comes on, and when it does, it doesn't stay on for as long as it used to, so I'm hoping they will eventually eliminate the problem altogether. I could be wrong, but this has been what I've been told and what I've experienced.
How much bigger can the ark down pipes be vs the stock ones. As far as I know it should be 2.5" it doesn't make sense. We don't have the 3" like the 2.0 guys do. It seems like crappy craftsmanship was involved instead.