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Discussion Starter #1
p0420 code keeps poping up on my 03 3.4 230k. but now my car, on acceleration chugs and cuts out on me when starting from a stop. it idles fine and when i start to drive the car it will drive fine for a while and then it will start chugging even at 55mph and i have to let off gas and accelerate slowly or i floor it and it will chug alot before it quits acting up. it doesnt do it all the time it does it every other time i drive it or after i have been driving for a half hour or so. some times it wont do it for a few hours and a few stops, some days its fine and some it is just horrible. the only code i got is p0420. my o2 sensors are functioning properly according to my computer. also in the same system is the catalytic converter. i was wanting to know if a bad cat actually would make the car chug like that. and or could my trans be making the engine do this? my trans slips a little bit here n there.
 

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Back in Black
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sounds like bad cat, how many miles on car and cat?
Same problem on my 2002 regal replaced cat runs better than new. If you have to replace cat do the ubend delete at same time, more power better gas mileage

not sure if the 3.4 has ubend I know the 3.8 does.
 

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Discussion Starter #4
sounds like bad cat, how many miles on car and cat?
Same problem on my 2002 regal replaced cat runs better than new. If you have to replace cat do the ubend delete at same time, more power better gas mileage

not sure if the 3.4 has ubend I know the 3.8 does.
230k miles original.
 

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LIFETIME HILLBILLY
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6,197 Posts
The 3400 does have a u bend. And that is a bad cat. I just recently fixed the same problem. I'm on my phone so I can't post the link. Search: "no more p0420 code :)" here on this forum. It's an easy cheap fix at a muffler shop, with a factory replacement cat. Mine was done for $57 total for cat and install. The cat will run you around 50 plus installation. I have links posted to where to buy the cat if the if your muffler shop doesn't carry it. It's the thunderbolt spun solution converter. Designed for the gm 3100, 3400, and 3800 engine. You will be blown away when its fixed. Around 6 months ago I priced a new cat installed and was quoted at $217 so I would jump on this if I were you.
 

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Mr. Handy
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6,289 Posts
I personally wouldn't just simply cut the original cat out and weld a new one in. The welds are weak points and eventually will rust or crack. And chances are the rest of the downpipe is on its way out as well. An aftermarket downpipe such as one from zzp will last much longer and will have the u-bend delete already. Just dont forget to add the option of an 02 bung and new gaskets. The downpipe replacement is an easy enough job to do yourself. 4 bolts and the o2 sensor, thats it. Replace the bolts while your at it.

ZZPerformance - ZZP Downpipes #ZZ-GPDP

$200 for an aluminized and $250 for SS
 

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OEM LOL
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1. zzp custom welds all there stuff together and alot of people stopped using them because of shitty welding.

2. zzp uses high flow cats that will most likely set a check engine light.
 

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I would not recommend an non GM cat to anyone, especially if your engine is higher mileage. Non GM cats have significantly less catalyst. There's a really good reason why a lot of of the non GM cats come with warnings about resolving the original issue that caused your factory cat to fail - it's because they rim job you with 10% of the factory catalyst amount, then 8 months later then the 420 code pops up again, they blame it on your engine, not their cat. Total waste of money, IMHO, but that's due to my own bad experiences. If you're tricking out or don't drive it much, that's one thing, but for daily driver/emission pass purposes, I've learned the hard way to stay as far away from them as humanly possible. I've.. heard...there are things you can do with the 02 sensor to work around it.
 

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OEM LOL
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I would not recommend an non GM cat to anyone, especially if your engine is higher mileage. Non GM cats have significantly less catalyst. There's a really good reason why a lot of of the non GM cats come with warnings about resolving the original issue that caused your factory cat to fail - it's because they rim job you with 10% of the factory catalyst amount, then 8 months later then the 420 code pops up again, they blame it on your engine, not their cat. Total waste of money, IMHO, but that's due to my own bad experiences. If you're tricking out or don't drive it much, that's one thing, but for daily driver/emission pass purposes, I've learned the hard way to stay as far away from them as humanly possible. I've.. heard...there are things you can do with the 02 sensor to work around it.
x2 on what he said

but you can run 02 simulators to get around the check engine light
 

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Mr. Handy
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Sounds like I might be going with a catless downpipe and an 02 sim then. I have never heard about people having problems with zzp's stuff. News to me
 

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Discussion Starter #12
How much are the simulaters?
 

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Mr. Handy
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OEM LOL
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thats what will at overkill did for me. He just removed the code.

as long as the dont hook the tube up to your exhaust you should be able to pass inspection.
 

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Mr. Handy
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No inspection here so no worries. I think im going to go with a cat'd downpipe w/ 02 bung and buy an 02 emulator and bung plug just in case I have issues down the line.
 

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Back in Black
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No inspection here so no worries. I think im going to go with a cat'd downpipe w/ 02 bung and buy an 02 emulator and bung plug just in case I have issues down the line.
Sound like a good plan and leaves you options:eek:k3:
 

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Mr. Handy
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^^ Thanks although it's not cost effective I will be covered
 
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