Thread: 03 gt p0420

Car tossed this same code almost a year ago and I was about to try the ZZP O2 Bung Extender but the light turned off and never came back on, well till now anyways. Car is running fine and when I fist pound the cat I don't get a rattle sound. No rotten egg smell either. Way I look at it I got 3 options. The bung, toss a o2 sensor at it, or get a ZZP PCM to shut off the rear o2 sensor. Think its worth it to try a sensor? Normally if its a sensor it toss the heater circuit code as well. Does the bung hurt MPG or drivability at all? ZZP says right on the site "We do not recommend running without the rear o2 sensor because it could run the rear o2 test and reduce your gas mileage" that worries me about going with the tune option.


So whats should I do? I guess buying a new cat is an option but a GM cat is like 700 bucks, Amazon shows an ACDelco AFS141 for like 33 bucks. I'm not sure if this is upstream or downstream. I think it's the down stream....

Originally Posted by Russosaur

Do you need a cat to pass emssions/inspections.

This. I mean if you don't need it then why bother.

Never. Lol. I have no cat and I don't smell raw fuel. Maybe the people behind me though lol

Yes I need it for PA inspection.

Originally Posted by Fahrenheit

Yes I need it for PA inspection.

Visually check plus plugged into the computer

Originally Posted by Russosaur

What all do they test/look for.

Well crap, your right now that I look at it. These cars have 2 sensors right? Might as well replace both at once. ACDelco AFS109 and ACDelco AFS141 right? 30 bucks a pop on amazon.

So im sorry if im being dumb but how many O2 sensors does my car have? 3 or 2? I was thinking 3, one on each exhaust bank and one post cat. Everyplace I google still says P0420 is a cat code. I got one of them scan tools that link to my phone, gonna check the O2 sensor output reading and see what that says

Thanks for the help. I just checked the volts on the o2 sensor. Upstream jumps from 0.1 to 0.9 while the downstream one sits at 0.3 to 0.4 and that's with the car warmed up at 2000 RPMs. When driving around I did notice the Upstream 02 will read 0.0. Not sure if that is an indication of anything. Downstream o2 does bounce around while driving but much less then upstream. I got a temp gun as well to do some additional testing.

The Upstream is moving pretty quick. Not sure how fast it should be moving.

Yeah it's moving like that. It do a full up and down sweep in like 2 seconds.