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Thread: Indication of Failing O2 Sensors?

  1. #1
    Advanced Tuner CCS86's Avatar
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    Indication of Failing O2 Sensors?

    Hi guys,

    I am seeing some weird O2 sensor behavior of my supercharged 2012 GT. It have had it repeat twice now, on different drives.


    Following a WOT blast, I go into decel and the WBs go full lean and the rear O2s fall to around 0v:

    O2-1.png


    About 7 seconds after running near 1.0 on the WB, one of the rear O2s wakes up. The other follows a couple seconds later.:

    O2-2.png


    20 something seconds later of the wideband bouncing between 1.01 and 0.99, they are still not switching. Just kind of lazily hanging around or under .75v

    O2-3.png



    Now, about a minute after the WOT ended, the STFTs flatline at 0.0, the rear O2s start switching actively at ~0.9v, and the widebands bounce between 0.98 and 0.88

    O2-4.png



    This seems to suggest that there is a ~10% disagreement between the front and rear O2s about where stoich is.

    Is this showing the ECM trying to recalibrate the wideband?

    Does this show that one or more of my O2s are failing?

    What are we looking at here?
    Attached Files Attached Files

  2. #2
    Advanced Tuner CCS86's Avatar
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    Really hoping someone can lend some insight here. Something doesn't seem right.

    Here are two different steady cruises. One is at about 2500 rpm and 44 mph, the other is 2700 rpm and 60 mph. The slower cruise has the rear O2's steady at 0.175v, the faster one at 0.76v. Neither are actually switching.

    o2-2500rpm.png
    O2-2700rpm.png


    It seems like it could be one of two things:

    1. The widebands have aged and are not accurate
    2. The narrow bands are old, lazy, and inaccurate


    Both sets are original and have ~47k mi. Is there some way to decipher between these two possibilities without installing an additional wideband, or blindly replacing? The price difference between the front and rears isn't huge. But, replacing all 4 would be a chunk.

  3. #3
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    The rear O2s shouldn't switch like front narrow bands or WBs. Stoich can be anywhere between .8 and .2 volts. Their main job is to tell the ECU the Cats are working. They should for the most part stay at a steady high voltage(rich/low oxygen). Dropping to a lower voltage indicates there is oxygen making it through the cats. This means CO and other unwanted gases are too.

    The fact that b1s2 and b2s2 are showing the exact same value makes me think the PID you have is not actually looking at two different sensors. Now is this HPTs scanner or the ECU using substituted values not really sure. Try another scan tool and compare.

    Yes the Rear O2s are used to keep the fronts in calibration. This happens all the time. Turn off the FAOSC and see if what you are seeing stops happening. It could also be learning the transport delay.

    For the most part your front O2s and rear O2 look to be reading correctly. I dont think you have any sensors failing or starting to fail. Theres nothing thats happening that couldn't or shouldn't be happening. You can see how the short term FTs keep the fronts at lambda and the rear O2 voltage is sensitive to the changes it makes.

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    Advanced Tuner CCS86's Avatar
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    Quote Originally Posted by murfie View Post
    The rear O2s shouldn't switch like front narrow bands or WBs. Stoich can be anywhere between .8 and .2 volts. Their main job is to tell the ECU the Cats are working. They should for the most part stay at a steady high voltage(rich/low oxygen). Dropping to a lower voltage indicates there is oxygen making it through the cats. This means CO and other unwanted gases are too.

    The fact that b1s2 and b2s2 are showing the exact same value makes me think the PID you have is not actually looking at two different sensors. Now is this HPTs scanner or the ECU using substituted values not really sure. Try another scan tool and compare.

    Yes the Rear O2s are used to keep the fronts in calibration. This happens all the time. Turn off the FAOSC and see if what you are seeing stops happening. It could also be learning the transport delay.

    For the most part your front O2s and rear O2 look to be reading correctly. I dont think you have any sensors failing or starting to fail. Theres nothing thats happening that couldn't or shouldn't be happening. You can see how the short term FTs keep the fronts at lambda and the rear O2 voltage is sensitive to the changes it makes.

    AF Basics 3.gif


    Hey Murphie,

    Good to know that it's normal to not see the rear O2s switching. Is this because they are set up to switch about a stoich of 14.64:1, but end up running very close to 14.08:1 (slightly rich)?

    I am definitely logging both rear O2v outputs, as the values differ quite a bit sometimes. Check out the actual log file I posted and you will see. Here is an example where at a steady cruise, one of the rears just falls off (seems to dispute the theory of staying at high voltage because of 14.08:1):

    Rear-O2-fall-off.png


    How can the rears be used to keep the WBs calibrated, unless they are made to actually switch? You can't really infer an accurate mixture from a steady narrow-band voltage, can you?

    Even when the ECM is making the widebands do this larger oscillation (about a 12% swing in mixture) the rears still do not switch:

    O2-Dance-2.png

  5. #5
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    The rear sensor is dampened by the catalytic converter. Ideally, it would be a flatline with no excess fuel or oxygen. It should not match the front if the converter is in working order.

    If you had no converter, the front and rear would track each other. If the front is cycling rich-lean-rich-lean then you are good. The rear is just telling you and the ECU the condition of the catalytic converter.
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    Quote Originally Posted by b4black View Post
    The rear sensor is dampened by the catalytic converter. Ideally, it would be a flatline with no excess fuel or oxygen. It should not match the front if the converter is in working order.

    If you had no converter, the front and rear would track each other. If the front is cycling rich-lean-rich-lean then you are good. The rear is just telling you and the ECU the condition of the catalytic converter.


    That makes a lot of sense, thanks!

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    Is it normal to see the WB signal drop to 0 on a semi-regular basis?

    WB-dropout.png

  8. #8
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    Normal see it all the time. Might just be OBD port data loss.
    "We can never be right, we can only be sure that we are wrong"- feynman

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    Quote Originally Posted by CCS86 View Post
    Is it normal to see the WB signal drop to 0 on a semi-regular basis?

    WB-dropout.png
    common happens all the time when logging..prob the scanner using it, but never an issue. When the o2 is dead it will read flat 0.00