my 99 ek hatchy sputters and farts after i put in a 95 ex header and a custom catback with an rsx s muffler

the code it throws is P0420 Catalyst System Efficiency Below Thresholdis that whats making it sputter and cut out or is there something else wrong and how do i fix it ?

jumper harness and p28 or 02 simulator??? thanks in advance guys

 

my bad it starts sputtering and jerking at about4-4.5k rpm and does it all the way to redline unless you let off the gas btw it is a 99 dx and yes they connected the first and second 02 onto the header thanks in advance guys

 

please help

 

mine did the same exact thing, it turned out to be that i needed new spark plugs and wires.

 

The most obvious Q would be: what cat are you running?