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Thread: DIY precat o2 sensor change

  1. #1
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    DIY precat o2 sensor change

    This is a DIY for the e39 M54B30.

    Tools needed. I would highly suggest that you purchase a 22mm o2 socket. I will make changing these so much easier.



    This is what I used. Bosch part #13477



    New vs Old.



    To start you will need to remove the coil cover. Not totally necessary to remove but it will give a little easier access to the sensors as well as a little better visibility.


    • Remove the two cover caps with your small flat edge screwdriver and put aside.
    • Remove the oil cap.





    • Using a 10mm socket and driver remove the 2 nuts.
    • Simply lift the coil cover off and place to the side.
    • Replace oil cap.




    Once the cover is out of the way locate the pre-cat o2 sensors. They are in the exhaust manifold. Bank 1 is the one closest to the front of the car and bank 2 is closest to the firewall.



    There is a wire cover that protects the wires of the o2 sensor. It is held in place by two plastic clasps. Using your small flat edge screwdriver, carefully pry the clasps apart and pull off.



    Once you have the wire cover off the sensor connector is also held in place by 2 very fragile plastic clasps. I broke one on each connector so be carefull when pulling these apart.



    Once you have the sensors disconnected then take your 22mm open ended wrench or sensor socket, if you have one, and remove the o2 sensors. The bank 1 sensor is the easy one. The bank 2 sensor is a bear. My 22mm wrench was too long to be able to get a good bite. Luckily, I was able to wiggle it on so I could at least break the sensor free. Once the sensor was loose, I used an 8 inch crescent wrench to further loosen it until I could unthread it by hand.



    Once you have the old sensors out, you simply screw the new sensors in their place and wrench them snug. The new sensors ship with antisieze on the threads in case you ever have to do this again.
    The connectors only plug in a certain way so make sure you have them oriented in the proper way.



    That's it. Reverse the steps above to reinstall and call it a day. If you have everything you need and don't run into any major catastrophes, this little job shouldn't take more than an hour to complete.
    Last edited by jblindy; 10-09-2011 at 09:20 AM. Reason: Added which engine this DIY is for. Very important stuff there.
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  2. #2
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    Nice write up. Man that looks way more involved than the 540. I think it took me longer to get the car up on ramps, get the one tool I needed (22mm box wrench) and shimmy under the car than it did to actually change the sensors out on mine. Still, all told, I think it only took 20 minutes or so.

  3. #3
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    Longest part about doing it on my 540 was locating them.
    Fyi- they are where the exhaust starts to curve upward towards the engine
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  4. #4
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    Before I even started doing the sensors, I got the car up on stands fully expecting to have to work under the car. So I gather all my tools and crawl under the car, I easily find the post cat but where in the hell are the pre cat? I'm looking and looking and not finding anything and then the dread starts to kick in, "Man these things are going to be in an impossible to reach location and this is going to take me all day!" So I crawl out and open the hood and much to my dismay they are right there on top. If I would have had the sensor socket it wouldn't have taken more than 20 minutes to do, but there just isn't much room for a 22mm wrench on that back sensor. The sensor was turned just enough that I couldn't get a good bite with the wrench. Close but I needed just a hair more. I finally got a hammer and slowly tapped it onto the sensor. Getting it on was just half the battle, now I had to figure out how to break it loose. I actually did it old skool style and climbed up on top of the engine so I could get better leverage. It worked. Once I got it broke loose, I had to use the 8" cresent cause there was no way to get the open end wrench on there. I got it though with a little persistance. I will never concede defeat! LOL
    Aut inveniam viam aut faciam

  5. #5
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    Thnks for sharing. Subscribed!
    Live the e39..!!


  6. #6
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    Nice write up. Now if I can only find the post cat DIY...

  7. #7
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    I currently writing the diy for lugnut removal.
    ~2001 540i/6speed~
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  8. #8
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    This is the pre-cat E39 M54 O2 sensor DIY I used today.
    What a damn good examplar: Explicit attention to detail and lots of pics.

    Using the crow's foot socket tool (from Autozone) was invaluable.
    I can't even imagine trying this with a wrench.
    Their slotted spark plug style wrench was too short.
    Once the seal was broken, I just had to be patient and do 1/8 of a turn at a time.
    I could never get my hand in there to unscrew manually. My back was feeling it.


    U-joint and misc. adapters & extension bars for my ratchet set also came in very handy.


    Yes, the plastic connectors were a pain in the ass to remove.
    I also broke that 2nd one when trying to unsnap using a screwdriver.
    I also used a dental tool to help pry it.
    In retrospect, I'm not sure if that made it harder


    I was afraid I would not be able to re-thread the rear O2 sensor since it was such a tight squeeze,
    but I was able to get it back on using both hands.

    I have no idea if I replaced them with enough torque, but I'd rather be loose than break/strip something.
    I just pushed it firm, and that was that. Should that be fine?

    Other gotchas that reminded me of working on cars in high school:
    I dropped the dental tool way down into the bay crevice where you work, and it took 15 mins to knock it free with a clothes hanger.
    That's the kind of stuff that's never in the Haynes manual!

    Total time was over 2 hours for a first time DIY.
    If I did it again, it would take half that.

    A huge thanks to this forum, in general.
    I am very happy to have gotten under the hood of a car again.
    Last edited by OnTheFence; 12-26-2011 at 06:23 PM.

  9. #9
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    Did my post-cats last year. Not easy to get too and you'll lose some skin.
    I imagine you were getting Cat related codes.
    Has your gas mileage improved? Does your engine idle better/run smoother?
    "I'd smash that (Jennifer Connelly) like a failed coup in sub-Saharan Africa."
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  10. #10
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    Actually, my light wasn't even on. I just bought this car with 120k and I wanted to try a few DIY's that are supposed to fail anyway. (I know you're not a fan of this) This is my daily driver, so I don't mind some preventative things while I have some days off. But, I figure $90 on my own time, or $500 if I'm in a pinch and can't do it for myself. And part of the reason I got a BMW was to learn to work on cars again.

    For what it's worth, this is the old sensor next to the new one.

  11. #11
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    Onthefence - I replaced my pre-cats twice, my car has 150K miles.

    Above, I had two sentences that ended in questionmarks. Can you speak to those questionmarks?
    "I'd smash that (Jennifer Connelly) like a failed coup in sub-Saharan Africa."
    ~Macktheknife in my epic Jennifer Connelly OT Thread

  12. #12
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    Well, how do you like this: The SES light (Service Engine Soon) came on AFTER doing the O2 sensors. I drove about 10 mins. Ugh, maybe I should have left them alone. I read another thread where the fuse might be out. Maybe the sensors were dead so the SES never came on? I will read the codes tomorrow. Glad I now get to use my code reader, I guess.

    5er: I didn't notice anything, as nothing was really wrong in the first place. Can't tell about gas mileage b/c my ABS module is out of the car right now.

  13. #13
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    Quote Originally Posted by OnTheFence View Post
    Well, how do you like this: The SES light (Service Engine Soon) came on AFTER doing the O2 sensors. I drove about 10 mins. Ugh, maybe I should have left them alone. I read another thread where the fuse might be out. Maybe the sensors were dead so the SES never came on? I will read the codes tomorrow. Glad I now get to use my code reader, I guess.

    5er: I didn't notice anything, as nothing was really wrong in the first place. Can't tell about gas mileage b/c my ABS module is out of the car right now.
    Did you cross-up the 02 plugs? Yes/no.
    Did you use Made in China crap sensors or OEM? Yes/no.
    On the I6 there is an air pump mounted to the block with a rubber vacuum hose attached to it...the hose is very close to the job you just did...check that hose.
    "I'd smash that (Jennifer Connelly) like a failed coup in sub-Saharan Africa."
    ~Macktheknife in my epic Jennifer Connelly OT Thread

  14. #14
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    I used the Amazon OEM Bosch plugs.

    What do you mean by "cross up the plugs"?
    I just bought 2 of them and put them in one at a time.
    And I believe they only plug in 1 way based on the shaped grooves.

    I will check for a loose hose tomorrow AM.
    I will also read the codes.

    Thanks for sharing your skills.

  15. #15
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    Quote Originally Posted by bimmerfiver View Post
    Has your gas mileage improved? Does your engine idle better/run smoother?
    Interesting question. I replaced my pre-cat O2 sensors last month at 104K and have seen a "noticeable" improvement in my gas mileage. I would estimate the improvement at better than 5%, which translates to about 1 mpg around town and 2 mpg on the highway, based on a comparison to my historical gas mileage. I had no codes prior and did this a PM. And I used Bosch OEM sensors.

  16. #16
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    I missed this DIY. Anyone know the torque values for the o2 sensors?

  17. #17
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    50 nm is what I am reading.

    Just changed my pre-cats, had to run out to get a 18" breaker bar in order to get them off.

    BTW, is it ok to just change the pre-cats sensors and not the post-cats. I am just changing the pre-cats as pm and have no issues.

    Jamesdc4 - this should go into the DIY sub-forum?
    Last edited by yersole; 10-22-2012 at 08:41 PM.

  18. #18
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    tagged i want to see what happens to On the fence

  19. #19
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    Advance Auto Parts Rents you the O2 socket for free.. Credit Card deposit full refund when returned,,

    Just listen to the sound of the engine. Isn't that the answer? If you don't understand it, it's not my problem". alex zanardi

  20. #20
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    now someone please upload my next step which is post-cats sensors

    also this socket do not work for post-cats , its too short

    Last edited by champaign777; 10-22-2012 at 10:49 PM.

  21. #21
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    Why do you change the post cats? And why is that crowfoot too short? Both pre and post cats have the same body. The wire length is different and the connectors are different.
    Looking for a DIY? Parts? Check this out, it might be your ticket

    Stable: e92is, e53 N62, e46M54B25, Tribby & e39 M54B30 R.I.P.

  22. #22
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    Quote Originally Posted by dumblucky View Post
    tagged i want to see what happens to On the fence
    This was my first DIY ! It went pretty smoothly. Lights went away shortly after, and may have been related to ABS module.
    Scariest part was not knowing if I could get the new O2 back in without cross threading. You need TINY hands!

  23. #23
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    I always add additional anti-seize to the sensor threads and mating surfaces.

  24. #24
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    Quote Originally Posted by OnTheFence View Post
    This was my first DIY ! It went pretty smoothly. Lights went away shortly after, and may have been related to ABS module.
    Scariest part was not knowing if I could get the new O2 back in without cross threading. You need TINY hands!
    Did you have a difficult time getting at the rear bank 2 sensor in the back with that 22MM wrench? I remember when I did mine I used the O2 sensor wrench with a 12" extension to get at it. I had to remove the rear cabin filter housing to get an angle on it. However, this is definitely a DIY maintenance item that I think pretty much everyone can do.

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  25. #25
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    Quote Originally Posted by Doru View Post
    Why do you change the post cats? And why is that crowfoot too short? Both pre and post cats have the same body. The wire length is different and the connectors are different.
    1 because i bought both , car is 10 years old why not replace all 4 ?
    2 because i already tried with short O2 Wrench to install post cats and it doesnt work so i stopped
    Need long O2 Wrench for post cats Doru

    Last edited by champaign777; 10-23-2012 at 05:34 PM.

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