Your P401 could be any number of things.

Clogged egr passages, bad DPFE or even a broken green vacum hose on the egr valve.

A bad cat would throw a P420

read these. link1 link2 link3 link4


1996 2.5LT V6 Contour ATX My Tour+help/info pics E0 rims, Gutted Pre-Cats,CTA Custom Exhaust & Intake, Diablo Chip.