Getaway_Driver

I recently paid a Chevy dealer $589 to fix my previous issues. I got the car back on Tuesday, drove 130 miles since then and the CEL came back on this morning. After putting my code reader on, I saw P0420.
A bit of research revealed it's "catalyst below efficiency."
I'm truly at a breaking point with this car. Can anyone please tell me what I need to resolve this issue?
The car is at 115,150 miles.
Around the 90k mile mark, I had a muffler tip installed on it. The shop just removed the stock exhaust tip and welded on a black one. It had nothing to do with performance. I just wanted a black exhaust tip.
Could this be the issue? I don't understand why it'd pop up nearly 30k miles later, though...
FWIW, I still have the stock exhaust tip sitting in my garage.

blueLNFftw

iTrader: (16)

Cat is clogged or the rear o2 is bad.

Getaway_Driver

Damn. Sounds possibly costly. I'm wondering if having a shop weld the old muffler back on will fix it..?

Nothing but issues with this car. And I'm so close to paying it off, too.

blueLNFftw

iTrader: (16)

Cat convertor.
It has nothing to do with the muffler.
This is under the car in the downpipe.

ECaulk

iTrader: (6)

Check the classified for a cat, or just buy a catless or hi-flow with a defouler and ignore the light or not have the light. It would def be cheaper than buying a new stock cat

Nevermind about the high flow or catless, just saw you're in Cali, look for a stock cat.

Getaway_Driver

Are there not any catless exhausts for the Cobalt that meet CA emission standards?
I just got back from the local Chevrolet dealership. They wanted $1575 for the part and $230 for labor.
Lol That's just insane.

ECaulk

iTrader: (6)

post a wanted add, might check the regulations to see if CARB will accept high flow cats, or have an exhaust shop weld in new cats

blueLNFftw

iTrader: (16)

His best bet is to buy someones stock LNF downpipe here.

I would have sold him mine if I had it still.

Snail_SS

iTrader: (3)

He should have an exhaust shop check his cat make sure they are good first before buying. Also could be a bad sensor that you should find on crateenginedepot

Getaway_Driver

Assuming I can't find a catalytic converter within a reasonable amount of time here (this is the year my SMOG check will be due), is it just the converter I need?

Found this one on Crate Engine Depot:
Cobalt LNF 08-10 Converter 25816005

$1181 instead of $1575 like the dealership wanted. Still more expensive than I can currently afford, though.

Getaway_Driver

Called a local shop and they said they'll check it out for me. Thanks for the heads up.

exninja

iTrader: (1)

I can send you my old sensor. The heater is bad but otherwise it works. That would tell you if it's the sensor or the cat

Snail_SS

iTrader: (3)

If you need a downpipe im sure you can find one here for 10× cheaper. Just post a wanted thread and someone should have one they are willing to sell

Getaway_Driver

Okay, I know this is a stupid question, but I'm still confused about what I need exactly. I mean, what should I make a "Wanted" thread about?
Downpipe? Catalytic converter?

exninja

iTrader: (1)

The catalytic converter is an integral part of the downpipe. The downpipe connects the output of the turbo to the rest of the exhaust. The catalytic converter (cat for short) looks like a bulge in the pipe. The job of the cat is to allow for any combustibles to finish combustion before exiting the tailpipe, thus reducing emissions. The O2 sensor monitors this process, and in your case is saying that the cat is not operating correctly.

exninja

iTrader: (1)

Many people on here replace the downpipe with one that doesn't have a cat. This allows better flow through the exhaust, but is more polluting. CA doesn't look too kindly on this. Nobody does really, but the tests in other states are either non-existent or much easier to fool.

Since people replace it, used stock pipes come up for sale here regularly.

Snail_SS

iTrader: (3)

Yeah you could either buy the cats and replace them to the downpipe you have or buy a used downpipe from someone else. Or make sure iys not your sensors going bad

Getaway_Driver

Thanks, everyone, for the answers.

Just one more (possibly) stupid question:
If I replaced the downpipe instead of the catalytic converter, would it resolve the issue? Or is this something where I would be risking too much by going downpipe over cat? I mean, is the safer bet picking the cat over the downpipe or either one is good?

exninja

iTrader: (1)

The cat is part of the downpipe, so unless you cut and weld, it's one-and-the-same. If you get a new downpipe, it will have a cat in it. If you get just a cat, you will cut your downpipe and weld the new cat on. The downpipe sans cat is just a tube.

However, if your problem turns out to be the sensor then changing the cat/downpipe won't help. It's probably not the sensor, but it's a very cheap test to make sure. If I have two possible causes to a problem and my only troubleshooting method is to replace, then it is prudent to choose the cheapest option first.

Snail_SS

iTrader: (3)

Yup what he said ^^ so pretty much if you go and buy just the cats you will have to open the downpipe remove the old cats or cat, put the ones in, weld the downpipe back up (i wouldnt just trust anyone to do that). If you just buy a downpipe from someone it will come with the cats inside just like the one you have in your car right now, but the cats on the one your getting should be good.

Best thing is to test the cats efficiency in a exhaudt shop and see if they can test your O2 sensors and what ever doesnt pass just replace either way your not looking at more than 300 bucks
O2 sensors for one maybe around ~90 bucks new by the looks of it shipped
Downpipe from a member here maybe $80 and up

blueLNFftw

iTrader: (16)

There is a downpipe with a factory cat install in the marketplace now....

Getaway_Driver

Are you referring to this one:
Link

He says he won't ship the DP. He's in Texas and I'm in SoCal, so I can't go pick it up.

Did you see anyone else selling one? I saw another, but the dude has a catless DP. You already know shitty CA emissions standards won't let me go catless.

Snail_SS

iTrader: (3)

You need an oem one. A high flow downpipe will cause you the same code and if you dont get the code you probably wont pass emissions since your on cali

Getaway_Driver

I love Cali, but this **** sucks. This would be a perfect time for others to get an aftermarket DP, but in my case, I have to stick with oem.
Thanks for all the help, everyone. I appreciate it.

Snail_SS

iTrader: (3)

No problem. But yeah ha even my k&n is illegal in cali lol