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Thread: O2 sensor ? Hopefully not cats

  1. #1
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    O2 sensor issues. Mechanic now says ECU or wiring harness




    I have been chasing o2 sensor codes on my 2009 BMW 328x drive. And I have brought it to several mechanics, and exhaust professionals.

    I just got the a 60 day inspection pass since it doesn’t pass emissions in the current state. I was told to replace the o2 sensors, so I did all of them besides one that looks to have been done, and says Germany right on it. Not to mention I ran several tests through ISTA, and all tests passed saying the sensor resistance is o.k.

    I was also told that if I replace the o2 sensor, and the check engine light comes back.. That I might have a bad ecu, or wiring harness.

    At first I thought this was all due to a rust exhaust flange that meets the downpipe, and mid pipe. An exhaust professional, and my mechanic both said there is no leak.

    I do smell, smoke, and hear a little hissing coming from the exhaust manifold. I’m not sure if this is part of regular operation, or if it could indeed be a leak of some sort.

    Here’s some pictures of the codes I’ve been getting related to the o2 sensors...
    Last edited by Bmwpower603; 10-12-2018 at 10:12 PM.

  2. #2
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    Anyone have suggestions ?

  3. #3
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    Oxygen sensor codes can be a result of other issues as opposed to faulty sensors. I see you’re using ISTA. Have you followed through on the test plan for one or both sensor codes?


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  4. #4
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    Yes. I’ve followed through with the test plans.

    Ista wanted me to replace all the o2s, so I did 3 of them. Because the 4th one, the error code o2(Looks brand new) and tested o.k. on ista resistance test. It also said check for leaks. I don’t have much experience with exhausts. So I brought it to an exhaust professional, as well as my mechanic who also told me to replace the pre cat o2. Both mechanics said no leaks. Even though the exhaust flange is rusted out.

    I don’t think they were very thorough with the exhaust leak check.. I’m not even sure how to test for leaks, or how much of a leak would cause these codes...

    I paid $5000 for this car. And just put about $1300 into it in parts alone. Genuine bmw Water pump, t stat, 3 o2 sensors, and a drive shaft.. Not to mention oil pan gasket was done recently, and there isn’t a single oil leak... But I’m now at the point where if it needs an ECU, or engine harness.. I’m going to have to part with this car. (I hate to do that) since it’s a pretty rare 6 speed LCI wagon.

    I can afford to keep it if I can get to the bottom of this...

  5. #5
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    It doesn’t take much of an exhaust leak to affect the O2 sensor readings.
    The best way to check for a leak is with a smoke machine but if you can’t find a shop with one, try this:
    Put the car up on a lift with someone inside driving it, use a mechanics stethoscope with the wand removed so you have just the tube, have someone restrict the tail pipes with gloved hands or bunched up rags so that you create back pressure in the system. Then listen in around suspect areas with the stethoscope. Check for tiny cracks on the seams of the catalytic converter as well as any joint areas.


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  6. #6
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    O2 sensor ? Hopefully not cats




    Have you checked carefully that either the front or the rear sensor leads haven’t been reversed during replacement. That is the first thing on the list of things to check on 2C31.

    2C2B is a code that is triggered because of 2C31 so don’t focus on it.

    The other thing that you could do once you verify correct lead connections is to actually swap both front and rear sensors from Bank 1 to Bank 2 and see if the fault moves. That would then indicate a faulty new sensor. You’ve checked the resistance on the sensor but that doesn’t rule out a faulty sensor that is simply processing incorrectly.

    If the fault doesn’t move, then I would suspect a hairline crack in the catalytic converter is skewing the reading of the rear sensor.




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    Last edited by bms5; 10-13-2018 at 06:53 AM.

  7. #7
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    Okay. I tried swapping the pre cat o2 sensors. The code came back, on the same bank.

    I brought it to a custom exhaust place to listen for leaks. . They found none..

    Is there any other way to check the exhaust for leaks?

    What else could this be? Bad Cats? Bad DME? Bad electrical harness?

    I’ve had many bmws. And have never had to chase an issue like this.
    I thought this was the issue. But the exhaust place said they heard no leaks. Is there any other way of check this for a leak?
    Last edited by Bmwpower603; 10-13-2018 at 12:43 PM.

  8. #8
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    Some shops will blow compressed air up the tailpipe and spray soapy water around the cats, heat shields and flanges. They can be very tricky to find. Are you near any of the BIMRS.ORG shops?


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  9. #9
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    Yes. I have 3D Auto works down the street from me.

    Ironically I made an appointment with them for Tuesday.

    I sprayed windex on the flange while the car was running. No bubbles.

    As far as the manifold goes, I can smell some exhaust fumes close to the manifold. Not sure if that means anything.

    I’m out of ideas at this point. I’m not sure how anyone is going to diagnose this. My mechanic said if it’s not o2 sensors, then it’s the ECU.... My friend who has worked on bmws for years said if there is no leaks it’s the ECU or wiring harness. .

    That would suck!!

  10. #10
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    Dan should be able to get to the bottom of the problem; the BIMRS group is an awesome resource for the BMW specialists when there is an unusual problem like you’re experiencing. Keep me posted....


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  11. #11
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    I talked to one of the most highly regarded bmw tuners/ race side tuner/ bmw professor/ developmental engineer in the world via email.

    He was the bmw program professor at mass bay. I spoke to him through email, and he told me it’s probably the whole exhaust system. And not to over complicate things, and don’t blame it on electronics. But he no longer does service work, strictly bmw tuning. His name is nick pavloski..

    So hopefully it’s exhaust related. And not a wiring harness. As I would assume that a wiring harness would be a nightmare to replace.

  12. #12
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    I brought the car to 3D Auto. They said that the voltage difference between blthe two pre cat o2 sensors, are to far off. And this is caused by the cats starting to become clogged. They went in with a scope, and observed the bank 1 cat starting to fall apart inside. They are starting to get clogged in the honey comb inside as well.

    The tech working at 3D suggested that I try cat cleaner, and if that doesn’t work to replace the cats.

    $1000 a piece at BMW.

    I was thinking about getting Active Autoworks catless exhaust manifold + a tune. But I’m not sure if emissions in NH will allow that. I think all o2 sensors need to be working, and have set readiness.
    Last edited by Bmwpower603; 10-16-2018 at 08:43 PM.

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