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M54 primary 02 sensors

Discussion in 'E39 (1997-2003)' started by Autohaus, Sep 23, 2008.

    Autohaus guest

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    My 01 525i with 121K has the original 02 sensors (all 4 of them :mad:). Yesterday morning, the CEL came on. I did a full maintenance this past March (full tune up, changed fluids, other various parts, etc...). I know, my fault for not changing the sensors back then. Now since I have heard the primary sensors need to be changed between 100K-120K, I assume they triggered the CEL since I am just north of 120K. Mike Miller wrote an article about this in either Roundel or Bimmer that the two post sensors (after the cat) last up to 150K. I will order them (the primary sensors) tomorrow and get them by Friday so a friend and I can install them this weekend. They seem easy to get too. How hard would the swap be? Do I need my mechanic to clear the code after they are installed or does driving a few miles clear out the code? Any help would be appreciated. BTW, I have only driven the car at most 10 miles since the light came on. This week its parked on my driveway as I am driving my wife's 325xi. I don't want to ruin my expen$ive cat :D.
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    mooseheadm5

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    It could be on for hundreds of reasons. We have lots of customers on their original O2 sensors with more miles than you. Find out why it is on then fix that problem before you go shooting parts at it and hoping that was the fix. It could easily be on for a loose gas cap or a vacuum leak or something like that. You can never assume anything about why that light is on.

    Autohaus guest

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    True, but at that mileage the primary sensors would need to be changed. I had a vacuum leak fixed that caused the light to be on before. I understand what you are saying that I should have my mechanic read the code first before ordering the sensors, however, I am gambling at that mileage with those originals on the car. Thanks for the heads up :D.

    Autohaus guest

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    Update. My mechanic will look at the car in the morning and hook it up to his computer/code reader to determine exactly what it is.
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    mooseheadm5

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    Good. It is actually not a huge gamble with those sensors. They should be replaced by mileage, but the car will tell you when they are due if you want to wait. The base program will run it in a spec that won't kill the cats, but I understand you being cautious, though. As far as the rear sensors, let them go until the CEL comes on. They are only there to tell the computer if the cats are working. No other reason than OBD2 regulations.

    Autohaus guest

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    So it would take me driving thousands of miles before any damage to the cats if I chose not to change the sensors (assuming they caused the light to be on)? Also, with bad O2 sensors, would I immediately notice poor fuel economy, hesitation or would that happen over time? Thanks for your help.
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    mooseheadm5

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    Usually, they start to get slow to respond to mixture changes made by the computer. The ECu is clever enough to know when they are getting old and has an "O2 sensor ageing" code. Beyond that, the car may start having a poor idle quality and slightly greater fuel consumption (too lean kills the cats faster than slightly too rich due to the heat.) By then the light should be on for the O2 sensor being faulty. Beware the O2 sensor code, though. It can be tripped if there is a vacuum leak or an evap system problem that causes the mixture to go beyond the range of the O2 sensors' ability to adjust (the ECU only allows a few % change to the mixture due to the O2 sensor readings.) Many people have changed O2 sensors only to find that the light comes back on soon afterward because of a vacuum leak. This is not as much of a problem on the newer cars because the ECUs are smarter, but is a good thing to know.

    Autohaus guest

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    Thanks! I will post on here what my mechanic tells me tomorrow. Now this may not mean anything, but I usually average about 28-30 mpg on the highway. Yesterday after the light came on, I drove on the highway (about 5 miles) and the mpg reading was in the low to mid 20's.
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    mooseheadm5

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    Now that is quite a problem. Sounds like it is possibly the MAF. O2 sensors should not be able to have that much effect on the mixture.

    Autohaus guest

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    Mass air flow sensor/Air flow meter? $300 from Bavauto as I checked. $220 from another site. Eh, we shall see if it is that.
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    mooseheadm5

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    Yup. Wouldn't be the first time that one of these has needed replacing. The clue would be that there would be an addative or multiplicative mixture code stored in the computer, or simply a too rich or too lean code for both "banks" of cylinders (1-3 and 4-6.) Your mechanic not work on BMW stuff much? Most of us don't like diagnosing problems for you to take back home and fix yourself. Not much money in it.

    Autohaus guest

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    I work for him on Saturday mornings detailing cars, shop cleanup and VERY LIGHT mechanical work (usually me helping him with a few things). He specializes in BMW's and he lets me buy my own parts and he has no problem installing them. I should invest in a Bentley.

    Autohaus guest

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    What would be the exact location of the MAF?
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    mooseheadm5

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    I see. Then he should steer you in the right direction. Anyhow, MAF is attached to the airbox. It usually has a 4 or 5 pin connector on it. You can replace it very easily (10 minutes or less.) He may have a test unit for you to swap in if there is a question, or if he gets parts from Worldpac, he can always return it if it is not the problem. The Bently manual is not cheap, but it is pretty gash darn complete.

    Autohaus guest

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    Thanks for your help today. I will let you know what the problem is when I find out tomorrow.

    Autohaus guest

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    Ok, so last night I start my car and the CEL isn't on (comes on for like 2 seconds as normal, then goes out). I'm like..ok? I drive around town before leaving it at the shop and no light. I drop it off, turn it off and start her back up, no CEL. My mechanic can still read what caused it to go off, as its stored in the DME. Once he calls me, I will post.

    Autohaus guest

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    UPDATE

    Cam sensor, intake side. Does the manufold have to be removed to get to this? Where is it located?
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    mooseheadm5

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    It had better come with BMW on it. The reason I said OE BMW only is the Febi/Bilstein parts are made in China and we have seen with our own eyes 3 failures (one out of the box, one within one day, one within one month) from them. All the OE BMW ones we have installed (they come from our supplier sealed in BMW boxes) are still working just fine.

    Autohaus guest

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    It lists the part with a BMW OEM number. I have ordered parts from them for about 10 years and are well known around the BMW community. I would doubt they are using cheap chinese parts. However, thanks for the heads up. They offer a 2 year warranty on parts so if it fails, I'll complain :mad:
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    mooseheadm5

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    They all have a BMW part number listed for reference. The Febi brand is usually good, but in this case, bad cheap Chinese stuff.

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