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post #1 of 7 (permalink) Old 03-05-2019, 02:56 PM Thread Starter
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P0420 code help

Newbie here with a nagging exhaust issue. My cobalt is a 2010 base coupe with the 2.2L. 117,000 miles. Had to replace a severely cracked exhaust system.... including the cat. I live in Maine so you know the roads suck in winter. Replaced the exhaust manifold in the summer... just had to do the rest of the exhaust system from the manifold back.

After the switch, a week into the new parts, all made by Walker Exhaust, new downstream O2 sensor because the old one didn't come out clean. Upper O2 is original. I get the p0420 trouble code. It's resetting the code each time at around 700 RPM according to the freeze frame data. Watching live data during the drive cycle shows no issues after code is sent to the dash. All tolerances are spot on. Reset the code and popped back up.

Will replacing the bank 1 O2 sensor fix this considering everything is brand new on my exhaust? Or do I need to look deeper at this than I already have?
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post #2 of 7 (permalink) Old 03-06-2019, 10:45 AM
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I'd unplug and replug any sensor that was removed or replaced, making sure the contacts are good and clean.

2010 LT 2.2 4 door
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post #3 of 7 (permalink) Old 03-06-2019, 11:34 AM
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Did you replace it with a factory converter because if you didn't you will most likely get the p0420 code because aftermarket cats don't flow the same as factory cats (they flow too much) which makes the o2 sensor toggle to fast and make the ecm set the p0420. A bad o2 sensor/sensor 2 not sensor 1 - sensor 2 is the one that monitors the converter. If you didn't fix all the exhaust leaks that can set a p0420 code as well and depending on how long you had the leaks and how severe the leaks were it is possible it f*cked up the new cat by running lean (adding fuel) if they weren't all fixed at the same time
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post #4 of 7 (permalink) Old 03-06-2019, 04:54 PM Thread Starter
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Sensor bank 2, aka downstream, was replaced. Cat isn't factory. Don't have $600 for that. Cat broke free from the resonator for 2 weeks before I could fix it. Goddamn that was a loud drive. Haha!

I can't weld for crap, so I replaced both ends. Good thing I did because I discovered a crack half way to the muffler I didn't know was there. Along with the flex pipe cracked behind the exhaust manifold. New exhaust has no leaks. So P0420 code doesn't make sense.

Bank 1 (upstream) hasn't been replaced as it came out of the old cat just fine. I had to replace bank 2 because it was basically seized into the old cat and didn't budge. Connections are clean and secured. Made sure of it before I installed the new cat.

What is maddening is it only sets the code at idle, 3 times now at a red light, halfway through the drive cycle.
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post #5 of 7 (permalink) Old 03-06-2019, 06:56 PM
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if the converter isnt a factory replacement one then thats probably what the issue is as long as everything else is in good shape
the front o2 wouldnt set a p0420 code as thats before the cat and is used for fuel trim calculation

Last edited by TCSS07; 03-06-2019 at 06:58 PM.
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post #6 of 7 (permalink) Old 03-07-2019, 05:49 AM
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would one of these help? https://www.amazon.com/Dorman-42009-.../dp/B000CO91FY

2010 LT 2.2 4 door
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post #7 of 7 (permalink) Old 03-07-2019, 08:06 PM
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ive personally never used a fouler as i have Hptuners so i can just disable the dtc's as needed but ive worked with vehicles with them that still has the p0420 code enabled and how they work is by slowing down the o2 toggling (to simulate the converter) because it moves it out of the direct exhaust flow so yes it can work but wont guarantee it will but for 7 bucks why not try right

Last edited by TCSS07; 03-07-2019 at 08:08 PM.
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