My car is throwing a P0420 CEL and idleing rough. I was told that the cat needs to be replaced since it's not burning the mixture right. I was also told by a guy who owns a '96 Monte Carlo that he had the same code and one of his injectors were leaking into the cylindar and it wasn't burning all of it off. He told me that if I were to change the plugs, one would be black from this bad mixture. and sure enough, when I changed my plugs the last time, cyl 2 plug was black and the rest were a greyish color. I know that all cars are differant, so I'm really wondering if anyone else had this trouble and what cured it.