I just bought a 2018 5.0 XLT. It came with an aftermarket cat back set up that was installed nicely. However I'm constantly getting this code for bank 1 & 2.... "Catalyst System Efficiency Below Threshold. Code P0420". I want to get rid of the check engine light. Other than going back to OEM or putting black tape on the dash is there a way to do this?

 

a cat back exhaust system shouldn't affect the o2 readings,

What codes are you getting

 

My bad... its not an O2 code. It says "Catalyst System Efficiency Below Threshold. Code P0420"

 

ah. okay. either way a catback system should have no affect on the cats

first thing i would try is the italian tune up, see if that changes things. some of the 5.0s from 18-20 are oil drinkers, its possible that that fouled the cat or it just needs to be taken out and driven

 

Buy it from a dealer? Take it back if you just bought it. Make them fix it.