ANY modifications to the stock cats is illegal. BUT, how is any inspector gonna know if high flows installed on a vehicle are aftermarket? You shouldn't have any problem with CEL's with high flows. I will say that on my Magnum, I was getting CEL's after a year of having LT's and high flows. I ended up having to modify the rear O2 sensors by installing spacers thus pulling them outta the center portion of the pipe and closer to the edge which eliminated the CEL's. This was when there were no O2 sims out for the magnum (now there is!!). With our trucks, there are tons of O2 sims out there and they are cheap and easy to install. Its well worth the gain in HP to switch out or just remove the cats!!