hey guys, need some help.

Last week I got a CEL P0420 and the car idles a lil rough.
- I have been running the 3" XTC DP with cat for over 6 months and never got a CEL but I just recently installed my 3" hahn but even that didnt trigger a CEL.
- As well I was getting a tune and never had any CEL's.
- A friend said I might have got some bad gas but Im on my second tank of a diff gas and still getting the rough idle and Im guessing the same code, (no scan tool but thinking im going to buy one tmrw)

Any advice from those who have had this code?
Possible problems???

- bad O2 sensor
- cat clogging
- exhaust manifold leak/crack

Any advice would be greatly appreciated

Well the p0420 is usually a catalyst low efficency code. This could be from a damaged cat or even just the after market cat.

ya^

your cat is on its way out. that would definately cause a rough idle as well...huge amounts of backpressure and youll notice a slouch in performance

It means your CAT sucks balls

every once in a while a post cat O2 sensor will cause a p0420 but there will usually be a O2 sensor code with it. Every p0420 code i've seen has ended up being a damaged cat.

fack i have the same cat, i better not get this problem, sorry to hear that pete, keep us posted.

Talked to my buddy who works at the dealer here in kingston and he said to bring it in. So we hooked it up to GM's scan/monitor tool and the only code was P0420. He said in his experience that is basically a damaged cat, just like everyone in here said.

He checked the o2 sensor's frequencies and the front o2 was fine but the rear is nearly flatlining. I started at 500 mv and just continued to drop down to 65 mv when I like it's supposed to hover around 200 mv (not sure) and with that the idling started to get rougher but didnt seem completely related.

He revved it up and it shot back up to 400 and came down some and than started dropping again.

Can someone log their rear o2 to see what the voltage is at idle after the cat is warmed up? Bottom line it looks like I need a new cat.

wow... I"m still running the same cat is you for like 8 months now... even ran a tank of C12 race gas through it and still haven't thrown a code

I've got a p0420 from my 2.5" XTC, but it idles and runs great - my cat is not clogged.

wow... I"m still running the same cat is you for like 8 months now... even ran a tank of C12 race gas through it and still haven't thrown a code

I have had mine for right around 8 months......

I've got a p0420 from my 2.5" XTC, but it idles and runs great - my cat is not clogged.

getting a code is one thing...could use a mil eliminator to solve that but the idle is what is bugging me.

yeah that is kinda wierd. reset the battery and see if someone can turn off the rear o2 sensor in Hp tuners and you should be ok

its been cleared 4 times now but as I mentioned the CEL is not really the concern its how the car is running.

no. dont unhook the battery it wont do anything.

if you have a damaged cat, your rear o2 sensor will show funny readings. you need a new cat, bottom line.

umm i just got this code tonight but the car runs good....

The best cat....

Is no cat ;)

your rear o2 sensor should flatline and never go above 1 volt. as far as what the voltage should be....i am not sure

can i monitor this on my aeroforce?

your rear o2 sensor should flatline and never go above 1 volt. as far as what the voltage should be....i am not sure

how so, The front O2 fluxes between 400 mv and 500 mv and the rear should be lower than that and steady but where it should be steady at, is my question. 1v is SUPER RICH and 0v is DEAD LEAN. Im not sure what ur trying to say.

ok my o2 voltage bank1 reads anywhere from 850 to 20 mvs, it flucuates, sometimes its at 750, 600, 820, 55, 65, 450, ect ect, is this normal?

I disabled this code for my car with hp tuners for my catless downpipe which made it throw that code about a day after install.

I am experiencing the same issue as of 3 weeks ago (getting the same code). The car's idle bounces all over the freaking place. I got a copy of the idle fix and did a table comparison and implemented via HP Tuners but that did not fix it. Disabling the codes obviously does nothing other then making the CEL go away and nothing more.

Here is a copy for anyones reference what a plugged cat does. Watch when I stop or pop it in netural.. (Requires HP Tuners)

http://www.hptuners.com/forum/attachment.php?attachmentid=4946&d=1175489887

It also makes it bog the car between shifts and in the high RPMS

well my flex pipe is leaking all over the damn place, at idle the amount of exhaust leaking out is unreal, so not only is my cobalt addiction dp rusty as hell now, but the flex pipe has mad issues, i think i found my issue....

code just came up today. car seems to be ok. 2nd gear for some reason seems sluggish

code just came up today. car seems to be ok. 2nd gear for some reason seems sluggish

I already got mine fixed, plugged cat, it was replaced along with a faulty o2 sensor. It was telling it to dump more fuel then it needed.