My check engine light went off yesterday i was P0420 Catalyst System Low Efficiency ECM, PCM does that mean i need a new cat or has anyone had this problem before any help is appreciated thanks

 

Do you have any changes to your down pipe?

Most high flow cats and dp with no cats set off that code.

 

i just got a downpipe off of someone on here but he said it has a magnaflow hiflow cat but i've had to had the flexpipe fixed on it already is been on my car for bout a month

 

Mine does the same thing all the time with my ZZP 3" Downpipe with highflow cat, I just reset it using my Aeroforce, but it only comes on when I am idling at a stoplight.

 

Yea thats when i went off for me too i was stopped at the light

 

yeah these cars don't like high flow dp's, just have someone either shut off the code, or put an extender on the 2nd 02. that's it

 

aftermarket cats sometimes cause these codes when nothing is wrong, to keep the light from coming on you can tune it out with hp tuners, purchase an 02 simulator or use the sparkplug anti-fouler trick. i dont have time to explain it but do a search for it if someone doesnt explain it before i get home. but also, there is a possibility that your cat is bad and the code is legit.