Replacing Catalytic Converter

[brrundon]

Hey all,

Kinda a car noob here. My car has been reading error code P0420 for a while. I know it has to be the cat going bad because i had a prominent misfire for a while before i got it fixed. Now i plan on replacing the cat with a straight pipe or flex pipe. Whatever would be the cheapest. My question is, does anyone know the dimensions of the pipe i will need? I have a 2008 Grand Prix base. Also any other ideas you guys could throw at me would be greatly appreciated. Thanks!!

[Below Average Joe]

Is there a loss in power? Does the exhaust smell like rotten eggs? If not, I doubt the cat is bad. If you do replace it, you will get that code still. You will need a non-fouler for the rear o2 or have the code tuned out. Your car will dump gas compensating and get bad MPH. Trust me I know, I am dealing with this right now on our 05.

[brrundon]

That's what i do not understand. It does not smell like rotten eggs, and there really is no "loss of power". Do you mean like electrical power or acceleration? It does lag when i am accelerating, it feels like something is holding it back. It wasn't like that before the problems started to occur. And i have looked into getting foulers when i replace it. And my MPG has definitely been suffering. I am not sure where to go or what to do. I refuse to take it to another shop. If the cat is not bad, what would you advise? ive heard that it could be the o2 sensors too, but i also heard that the o2 sensors have to be working to throw that code. I am so misguided lol.

[Below Average Joe]

I meant loss or acceleration. sounds like the cat is going bad though. I would just straight pie it or buy a DP from ZZP or look in the for sale section for a cheap DP. I know where one is for sale, but I'm holding out.

[brrundon]

Which goes to my main question, any idea what size pipe i would need? I have been scouring for what seems like an eternity and i can not find a straight answer. I've seen 2.25", 2" and 2.5". Like i said, total car noob. I am not going to get a new cat, i would rather go the straight pipe or DP method, which ever works better and won't cost an arm and a leg.

[Below Average Joe]

It is 2.5" and a SP will be cheaper for sure. If you want a DP I can hook you up with a person who has one that might be for sale.

[Frosty]

I have deleted the cat on a couple 01+ cars without major issues. Non fouler trick is the easy way and it works well if you don't have a tuner. I personally use the zzp emulator which has worked flawlessly for the last 3 years for me. I have done the good old fashioned disable every single rear o2 code method as well and it will just go rich for 10 seconds on the highway to try and test and then after it fails all is well.

[brrundon]

Okay, awesome, thank you so much for the quick feedback guys! So, when i get out of work, i am going to grab a 2.5" straight pipe. Any specific length i should get?

[Below Average Joe]

As long as you need to fill the gap. I'd measure. How do you plan on securing this piece of pipe?

[brrundon]

I was going to have it welded in, so i wanted to make sure it was the right size. So i guess ill have to measure that bad boy

[Below Average Joe]

the shop you have weld it in should have pipe there. they will measure, cut and weld for pretty cheap hopefully.

[HighOctaneRacing]

the shop you have weld it in should have pipe there. they will measure, cut and weld for pretty cheap hopefully.

The shop may not even cut out the cat because it is considered illegal. I would spend the coin and get a ZZP catless down pipe with a bung so you can do the non-fouler trick to avoid getting the P0420 code.

The ZZP o2 emulater works flawlessy but it does not work on 04+ models and a 2008 GP cannot be tuned to remove the P0420 code because they are locked down from the factory. basically the only thing that will work correctly is the non-fouler trick. It's very cheap and easy to do provided you have the o2 bung.

[Frosty]

The shop may not even cut out the cat because it is considered illegal. I would spend the coin and get a ZZP catless down pipe with a bung so you can do the non-fouler trick to avoid getting the P0420 code.

The ZZP o2 emulater works flawlessy but it does not work on 04+ models and a 2008 GP cannot be tuned to remove the P0420 code because they are locked down from the factory. basically the only thing that will work correctly is the non-fouler trick. It's very cheap and easy to do provided you have the o2 bung.

I always wondered... Why doesn't it work in the 2004 gp?

[Russosaur]

I think it actually causes problems with all 01+ GPs because of the cat diagnostics it can loop the car in that mode and causing it to run stupid rich. I don't think its just 04+ specific.

[HighOctaneRacing]

I think it actually causes problems with all 01+ GPs because of the cat diagnostics it can loop the car in that mode and causing it to run stupid rich. I don't think its just 04+ specific.

Worked perfectly fine in the wife's 03 GP and she's going to be transfering the emulator to her 02 GP. IIRC, it doesn't work with the 04+ years due to the PCM not understanding the signal from the emulator

[SnowTh1ef]

Isn't the plug different on 04+?

[HighOctaneRacing]

I think it's still a square 4-wire design

[SnowTh1ef]

This note on zzp "* The rear 04+ Grand Prix sensor is the square style plug, we do not have the trapezoid shaped plug o2 sensor available yet." Is what I was thinking of I guess. Maybe the front is different?

[HighOctaneRacing]

This note on zzp "* The rear 04+ Grand Prix sensor is the square style plug, we do not have the trapezoid shaped plug o2 sensor available yet." Is what I was thinking of I guess. Maybe the front is different?

WTH? both my o2 sensors were squared on my 97 and so is my 06

[SnowTh1ef]

idk we all know zzp if FOS lol