vehicle is an 04 grand prix, 3.8L S/C... runs good and well maintained but i have a cat efficiency below threshold code... ummm, don't know what cylinder but it's front passenger side on the exhaust manifold is turning white and i can't get rid of the code... think the cat is bad but i'm not sure what would cause my exhaust manifold to turn white? running lean maybe? too hot? gas mileage has dropped to 16-17 as well... thats down from 20-22... vehicle has no mods. any help? thx!