Quote Originally Posted by WingZero_ View Post
+1 on the cat being the problem.



Wrong P0421 does not cause any type of driving/idling problems. Any car that has a header and no fix has this CEL. A secondary O2 sensor merely tells when your cat is acting up, or in a header case, not there.
Are you still going on about the header... I was just trying to help, if all you do is chirp people, why are you on the forum? suggest a solution.