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Thread: O2 Sensor Wiring - Pre-Cat

  1. #1
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    O2 Sensor Wiring - Pre-Cat

    Need some clarity on O2 sensor wiring and having some challenges finding anything thru searches here an other forums.

    The pre-cat sensors are both connected to the engine chassis at the fuel rail. I swapped my motor into a new chassis S52 to M44. Complete engine and body harness was moved over, so there's no mixing of components - all S52 electronics were moved to a bare chassis. In the process, I installed new headers and am wondering if I may have swapped the B1S1 and B2S1 connectors while moving the 02 sensors to the new headers. Before the engine swap, I marked the connectors with zip ties to make sure I had them correct, but didn't account for the header install.

    I know where the bank/sensor is at the 02 sensor, but how can you find the harness connector that matches? I looked thru my ETK and I'm not seeing clear distinction of the connector to bank, although each connector seems to have a different number - ETK doesn't reference where in the ECU the bank/sensor is assigned.

    My harness is not setup in the original layout so where it sits front/back on the engine isn't much help. I'm starting to think the only way to confirm which is which is to run diagnostics with one of the O2 sensors disconnected to see which throws a complete failure at the ECU readings. Any easier way to figure out which is which at the engine harness side?

  2. #2
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    Either disconnect one and diagnose the dme to find which was disconnected or trace the wires to dme pinout.
    -Abel

    - E36 328is ~210-220whp: Lots of Mods.
    - 2000 Z3: Many Mods.
    - 2003 VW Jetta TDI Manual 47-50mpg
    - 1999 S52 Estoril M Coupe
    - 2014 328d Wagon, self-tuned, 270hp/430ft-lbs
    - 2019 M2 Competition, self-tuned, 550hp

  3. #3
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    Quote Originally Posted by 328 Power 04 View Post
    Either disconnect one and diagnose the dme to find which was disconnected or trace the wires to dme pinout.
    One of the reasons I'm even thinking I have an issue is that I'm getting fuel trim issues so only now getting familiar with looking at the diagnostic data. What would I expect to see for the O2 sensor disconnected? If one or both are bad, should I still see which is disconnected? Smoke tests and FP isn't an issue, O2 has been coming up as an issue, but wondering if its due to swapped connectors vs bad O2 sensors.

  4. #4
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    You will have an error for oxygen sensor disconnected (or Oxygen sensor heater codes) etc. and it will say which bank.

    - - - Updated - - -

    Quote Originally Posted by modifiede30 View Post
    One of the reasons I'm even thinking I have an issue is that I'm getting fuel trim issues so only now getting familiar with looking at the diagnostic data. What would I expect to see for the O2 sensor disconnected? If one or both are bad, should I still see which is disconnected? Smoke tests and FP isn't an issue, O2 has been coming up as an issue, but wondering if its due to swapped connectors vs bad O2 sensors.
    Explain this a bit better
    -Abel

    - E36 328is ~210-220whp: Lots of Mods.
    - 2000 Z3: Many Mods.
    - 2003 VW Jetta TDI Manual 47-50mpg
    - 1999 S52 Estoril M Coupe
    - 2014 328d Wagon, self-tuned, 270hp/430ft-lbs
    - 2019 M2 Competition, self-tuned, 550hp

  5. #5
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    Quote Originally Posted by 328 Power 04 View Post
    You will have an error for oxygen sensor disconnected (or Oxygen sensor heater codes) etc. and it will say which bank.

    - - - Updated - - -



    Explain this a bit better
    I'll provide a more sequential list of what's happened and car setup that may help.


    Car is a track prepped S52 - M50 manifold, SAP delete, 3/2 delete, EWS delete, Cat delete, 3.5" 806 MAF, headers - internals are all stock currently.

    Initial start-up after the build was smooth but I soon experienced a MAF error, which I cleared after bridging the ground wires
    After clearing the MAF error, I started to get fuel bank errors.
    I ruled out vacuum after extensive smoke testing so am now at testing the O2 sensors and worried I've switched them.
    One issue I'm struggling with is that the ECU isn't storing values due to my kill switch - I seem to be losing code history - error the coding seems to change and my check engine comes and goes.

    The most consistent error seems to point to a potential O2/fuel bank error. I have no reason to believe its a fuel issue given I'm seeing the right fuel pressures at the rail (may check injectors if O2 isn't the issue)
    I'm awaiting a more advanced scanning tool to confirm fuel bank info and look at detailed live data from each system.

    Before I get my new scanning system I am mainly hoping to confirm I don't' have the wires switched.
    Last edited by modifiede30; 11-30-2021 at 10:44 PM.

  6. #6
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    Quote Originally Posted by modifiede30 View Post
    I'll provide a more sequential list of what's happened and car setup that may help.


    Car is a track prepped S52 - M50 manifold, SAP delete, 3/2 delete, EWS delete, Cat delete, 3.5" 806 MAF, headers - internals are all stock currently.

    Initial start-up after the build was smooth but I soon experienced a MAF error, which I cleared after bridging the ground wires
    After clearing the MAF error, I started to get fuel bank errors.
    I ruled out vacuum after extensive smoke testing so am now at testing the O2 sensors and worried I've switched them.
    One issue I'm struggling with is that the ECU isn't storing values due to my kill switch - I seem to be losing code history - error the coding seems to change and my check engine comes and goes.

    The most consistent error seems to point to a potential O2/fuel bank error. I have no reason to believe its a fuel issue given I'm seeing the right fuel pressures at the rail (may check injectors if O2 isn't the issue)
    I'm awaiting a more advanced scanning tool to confirm fuel bank info and look at detailed live data from each system.

    Before I get my new scanning system I am mainly hoping to confirm I don't' have the wires switched.
    I'm wondering if when you went to an 3.5" MAF, if the tune wasn't adjusted for it that maybe that could be an issue.

  7. #7
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    Quote Originally Posted by IndianaRoadster View Post
    I'm wondering if when you went to an 3.5" MAF, if the tune wasn't adjusted for it that maybe that could be an issue.
    No, the ECU was specifically tuned for the MAF.

    Sent from my Pixel 3 XL using Tapatalk

  8. #8
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    Quote Originally Posted by modifiede30 View Post
    I'll provide a more sequential list of what's happened and car setup that may help.


    Car is a track prepped S52 - M50 manifold, SAP delete, 3/2 delete, EWS delete, Cat delete, 3.5" 806 MAF, headers - internals are all stock currently.

    Initial start-up after the build was smooth but I soon experienced a MAF error, which I cleared after bridging the ground wires
    After clearing the MAF error, I started to get fuel bank errors.
    I ruled out vacuum after extensive smoke testing so am now at testing the O2 sensors and worried I've switched them.
    One issue I'm struggling with is that the ECU isn't storing values due to my kill switch - I seem to be losing code history - error the coding seems to change and my check engine comes and goes.

    The most consistent error seems to point to a potential O2/fuel bank error. I have no reason to believe its a fuel issue given I'm seeing the right fuel pressures at the rail (may check injectors if O2 isn't the issue)
    I'm awaiting a more advanced scanning tool to confirm fuel bank info and look at detailed live data from each system.

    Before I get my new scanning system I am mainly hoping to confirm I don't' have the wires switched.
    Can you post the exact errors you are getting?

    Also fuel trims would help a lot.




    I thought this felt familiar https://www.bimmerforums.com/forum/s...tial-A-F-Issue

    Well you made it to the right place.



    By the way, kill switches will not erase codes or clear adaptations in this DME, they're stored in flash memory that doesn't erase on its own. Codes that come and go means that the issue is on the fence of almost ok, and it "self heals".
    Last edited by 328 Power 04; 12-01-2021 at 12:50 PM.
    -Abel

    - E36 328is ~210-220whp: Lots of Mods.
    - 2000 Z3: Many Mods.
    - 2003 VW Jetta TDI Manual 47-50mpg
    - 1999 S52 Estoril M Coupe
    - 2014 328d Wagon, self-tuned, 270hp/430ft-lbs
    - 2019 M2 Competition, self-tuned, 550hp

  9. #9
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    Quote Originally Posted by 328 Power 04 View Post
    Can you post the exact errors you are getting?

    Also fuel trims would help a lot.




    I thought this felt familiar https://www.bimmerforums.com/forum/s...tial-A-F-Issue

    Well you made it to the right place.



    By the way, kill switches will not erase codes or clear adaptations in this DME, they're stored in flash memory that doesn't erase on its own. Codes that come and go means that the issue is on the fence of almost ok, and it "self heals".
    I'll post them shortly - hoping to get a more advanced reading on the fuel trims thru data logs so will post once I get it all together.

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