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Thread: E39 lambda / oxygen sensors

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  1. #1

    E39 lambda / oxygen sensors

    Why does the engine need both pre and post cat O2 sensors? AFAIK the pre ones talk to the ecu and help keep the air/fuel ratio in check, what are the after cat ones for then?

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  2. #2
    BMWHaus Guru
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    The front two monitor the AFR for the DME and the rears monitor the CATS.


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  3. #3
    What do you think of the notion that they should be changed at 100k?

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  4. #4
    BMWHaus Guru
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    Mine were gone at 80.


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  5. #5
    BMWHaus Contributor
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    If I remember correctly I got bosch sensors for an m3 at an okish price from euro car parts. I would imagine they would be the same sensor. Plenty of crud on the old ones so I could see why new ones would be a benefit.

  6. #6
    BMWHaus Guru
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    I did the same, bought Bosch units on amazon for mine and they were a fraction of the price of OE


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  7. #7
    Notice much of a difference after?

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  8. #8
    BMWHaus Guru
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    Just a smoother and cleaner power delivery, emissions were much better too.


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  9. #9
    Had you lights on the dash before?

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  10. #10
    BMWHaus Guru
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    No, but it failed the NCT and the readings seemed out on INPA


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