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2011 Malibu LTZ 3.6L V6 Red Jewel Tintcoat
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Discussion Starter #1
As title says, 2011 Malibu 3.6L V6. Additional info:
  • 141,xxx miles on car but only 47,xxx on engine (replaced at 94,xxx under warranty) - unsure if cats are original or if they were replaced with the engine
  • I almost always use Top Tier fuel
  • Oil is changed regularly using full synthetic
  • No known driveability issues or exhaust leaks
  • No oil consumption that I'm aware of
I just had all four O2 sensors replaced after getting the dreaded P0420 code. I replaced the "easy" one about a month ago but the code persisted. I bought the other 3 and had a shop replace those yesterday. All 4 are OEM AC Delco, bought from Rock Auto, so I'm sure that they're good stuff and are delivering good info. (I base that on what I've read on CMF.)

After picking the car up from local shop, I drove fewer than 15 miles and the CEL came on again, with P0420. My Torque Pro app describes it as "catalyst system efficiency below threshold (bank 1)." To me, that suggests that it's the one between the engine and radiator, but I'm not sure.

Would someone be kind enough to confirm or correct my information, just to be sure I don't buy and install a new cat, please? On RA they run over $200 each, and I just paid more than that in labor alone for the sensor replacement!

Another question would be if I can reset the code again and see if it stays off, hoping that maybe the cat has cleared itself.

If I were to watch the LTFT and O2 sensors, what would I be looking for?

Can I use any kind of chemical to clean the cats by adding to the gas or adding it through the fuel rail?

Thanks in advance!
DD
 

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Cat efficiency is measured by the sensor after the cat, fuel mixture is done by the before cat sensor. I'm still learning fuel trims so I can't give any advise there. They used to make after cat sensor eliminators to stop the CEL from coming on, I haven't used one in quite a few years though, to get through inspection.
Before cat sensor will move up/down as it controls fuel, after cat should read relatively flat as far as I know.
 

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With a scanner you want to watch the O2 sensor signals. Once warm, the upstream sensor, bank one sensor one, should oscillate up and down .2 v to .9 v every second or so, the down stream sensor [O2 bank one sensor two ] should not move very much. This is because the converter is supposed to absorb the extra fuel fluctuations. If the second sensor fluctuates to much, you get the P0420 code. Check the fuel trims but they are probably fine. Under %10 is good.

I suggest you do a very through job of looking for exhaust system leaks any where within 2 feet of the second O2 sensor, even down stream of the sensor. Leaks in a flex joint are very hard to find as it may test well at idle but when under load the motor moves and bends the flex joint. There are youtube videos and exhaust leak detection.
 

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2011 Malibu LTZ 3.6L V6 Red Jewel Tintcoat
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Discussion Starter #4
Thank you both! I'll report back with what I find, even if it's nothing...
 

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2011 Malibu LTZ 3.6L V6 Red Jewel Tintcoat
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Well, so far I have good news and I hope it stays that way!

I reset the CEL last night (Friday) after arriving home, and today (Saturday) it has remained off.

Here are 2 screen shots from Torque Pro. Revs around 2500, fully warmed up, and ...

67704


67705
 

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I am no expert on this subject but the numbers don't look to bad. The second O2 sensor are supposed to be hanging around .7 volt, which they are but I would have expected the 1st O2 sensors to oscillate at little higher and lower. Then again every vehicle is different. It appears you did this test while the vehicle was stationary. Better information would be presented during a drive with the engine under load. What you don't want to see is large fluctuations in O2 sensor number 2, although under large throttle plate movements this is inevitable. The computer monitors these voltage changes and has a factory set allowance or limits over which they are not exceed.

In some ways it does not make sense that new O2 sensors would correct this problem as new sensors are more sensitive and react faster to what they are experiencing in the exhaust system. I would think that If an O2 sensor was dead, a code would be set, but not for converter inefficiency.

All of this aside, with the code gone, just keep driving.
 

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2011 Malibu LTZ 3.6L V6 Red Jewel Tintcoat
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Discussion Starter #7
Thanks for that, and it makes sense.

I'll try to get a screen shot when I'm out running around today.

And like you say, so long as the code's gone and stays gone, I'm gonna keep on driving and hope it stays that way.
 

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Bank 1 is on the other side of the engine...between engine and firewall.
 

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2011 Malibu LTZ 3.6L V6 Red Jewel Tintcoat
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Discussion Starter #9
Oh, dang! I had it backwards. Thanks!
 

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Discussion Starter #11
Well, here's an update from driving it for a couple of weeks.

I can reset the CEL for the inefficient cat, bank 1, but in a day or less it'll be back on. Considering that all I drive is 11 miles to work, that's not a lot of miles before it returns.

Where I am at this point is I need to be considering replacing all of the cats, and that's not gonna be cheap or easy. I dread the idea, the expense, and the work involved.

If anyone has any ideas that could help in any way (cheaper, easier, etc), I'm all ears!
 

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Did you thoroughly check the exhaust system for leaks near the second O2 sensor bank 1 ? The flex joints in the exhaust can hide a leak while at idle and only leak while under load. The leak can be down stream of the O2 sensor and still cause a problem.
 

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No, I did not. Thanks for reminding me. I'll do that for sure, because if all it is is a flex joint or some exhaust leak, that's doable.
 
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