the ses light came on in my car, so i took it to the dealer and got it checked and they said it had a plugged up cat. the code it has is p0420, so my question is can i fix the problem by replacing the stock y-pipe w/ a aftermarket one or is the cat attached to the manifold?

 

the maxima has two cats. the precat and the main cat. if the problem lies in the precat you can just replace the y-pipe and, if you go aftermarket, eliminate it altogether. if the problem lies in the main cat just replace it with a highflow cat.

-i just posted a P0420 post about an hour ago so keep an eye on that.