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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 09-22-2012, 08:42 PM
scomike scomike is offline
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Shoul I replace my oxygen sensors?

I have replaced my cats should I have replaced my oxygen sensors?
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  #2  
Old 09-22-2012, 09:10 PM
charlie boy charlie boy is offline
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I would, do faulty o2 sensors contribute to cats being faulty? I would..
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  #3  
Old 09-22-2012, 09:42 PM
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ztom ztom is offline
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Get a code reader that can graph over time, so you can watch the o2s.
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  #4  
Old 09-23-2012, 05:56 AM
rdl rdl is offline
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Both BMW and Bosch who make the O2 sensors recommend replacing them at 100k miles; pre-cat and after-cat.

As the sensors age, they slow down in reporting mixture being rich or lean. The result is that the DME isn't able to optimize air-fuel mixture control. Several recent posts in the forums have reported a MPG improvement ~5% and better acceleration after replacing aged pre-cat O2 sensors. Bosch mentions MPG improvements only; silent on performance effects.

Many people replace the pre-cat sensors only. The theory being that the after-cat sensors are used only to measure catalytic converter efficiency to set an SES/CEL/MIL warning when a catalytic converter has aged enough to become ineffective. Other people disagree and believe that the after-cat sensors are also used by the DME for mixture control. I've not seen any BMW documentation that explicitly answers the question YES/NO one way or the other; there are only ambiguous hints and inferences. Except of course, for the genaral recommendation to replace all at 100k miles.

If the cats have been damaged by poisoning from contaminants such as lead, silicone (most often from head gasket leaks of anti-freeze into the combustion chamber or RTV sealants not marked as "sensor safe"), sulphur, etc. the O2 sensors will almost certainly be poisoned as well. If the cats were replaced due to physical breakage of the ceramic matrix inside the cat structure, the O2 sensors are most likely OK unless they are reporting a DTC.
EDIT: however since the sensor will be out & easily inspectable, google for "o2 sensor poisoning" and similar. Several sites will turn up with descriptions and pictures.

I've not seen any authoritative recommendation regards O2 sensor replacement if the cats were replaced due to aging effects and loss of effectiveness, i.e. no poisoning or internal breakag/damage. Personally, I'd apply the 100k mile guideline. But I've no technical data to support that opinion.
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Last edited by rdl; 09-23-2012 at 06:03 AM. Reason: sensor inspection
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  #5  
Old 09-23-2012, 08:44 AM
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ztom ztom is offline
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+1 100k miles
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  #6  
Old 09-23-2012, 10:55 AM
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champaign777 champaign777 is offline
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+1 60k+ miles as a preventive maintenance
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  #7  
Old 09-23-2012, 12:24 PM
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540 M-Sport 540 M-Sport is offline
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Quote:
Originally Posted by ztom View Post
Get a code reader that can graph over time, so you can watch the o2s.
+1, graphing will tell you exactly how they are working and whether replacement is required.

Otherwise, I would agree that replacement of the pre catalyst O2 sensors is sensible at the 100k point. Catalyst manufacturers will always recommend replacement of the pre-catalyst O2 sensors with cat replacement, only to ensure no damage is done to the new catalyst. Not sure that is really necessary if you can confirm yours is working well. Understand that if the O2 sensors were in seriously bad shape, they would trip a "CE" or "service engine soon" light.

*Note that old O2 sensors can get "lazy" in their response, and work at a slightly less than ideal operation, but not trip the "check engine" or "service engine soon" light. So some folks like to replace them early or preventively (if they have no means to confirm they are working optimally) so as to maximize fuel economy and performance.
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  #8  
Old 09-23-2012, 08:12 PM
lovemy98 528i lovemy98 528i is offline
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I had a check engine light and changed both up stream o2 sensors and one down stream sensor so a total of 3 and the check engine light went away the next day I started the car and now it runs like a beast! Faster and smoother acceleration I'm getting after replacing those 3.
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  #9  
Old 09-24-2012, 05:28 AM
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Fudman Fudman is offline
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I replaced my pre-cat O2 sensors at 103K and got better 5% gas mileage and smoother acceleration. Almost post Vanos-like. This is highly recommended.
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  #10  
Old 09-24-2012, 09:32 AM
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bluebee bluebee is offline
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I still have my original O2 sensors at about 120K miles. I haven't 'graphed' them but I looked up how to 'test' them in the bestlinks, and only found this.

- Oxygen sensor DIYs (540 M62) (525 M54) & locations (1) & O2 sensor tools you can make at home (1) or borrow for free (1) & all about 02 sensors (pdf) (1) & how to eliminate the rear o2 sensors (1) (2)

I didn't look in the Bentleys yet - but I wonder aloud if I should test mine (when I get a round tuit).
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See also: E39 Bestlinks & How to easily find what you need
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  #11  
Old 09-24-2012, 11:01 AM
lovemy98 528i lovemy98 528i is offline
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Honestly I'd just change them. You'll notice a difference in power.
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  #12  
Old 09-24-2012, 12:23 PM
OntheRock OntheRock is offline
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Related question... I recently ran out of gas and it triggered my "service engine soon" light and now having it checked out it appears that the cats need to be replaced ( this also corresponds with something I read in the manual that said that running out of gas could result in catalytic converter damage ) how does this happen ? or could it just be my sensors that need to be replaced ?

the car runs fine/perfectly otherwise
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  #13  
Old 09-24-2012, 12:53 PM
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540 M-Sport 540 M-Sport is offline
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Quote:
Originally Posted by OntheRock View Post
Related question... I recently ran out of gas and it triggered my "service engine soon" light and now having it checked out it appears that the cats need to be replaced ( this also corresponds with something I read in the manual that said that running out of gas could result in catalytic converter damage ) how does this happen ? or could it just be my sensors that need to be replaced ?

the car runs fine/perfectly otherwise
Clear the error, fill with gas and drive on. If the light comes back on, then you need to perform more diagnostics. Again, anyone with a proper scan tool that can graph the O2 sensor outputs can quick determine if you cats are truly bad, or it is simply bad post cat O2 sensors.

Cats rarely fail, and I am unable to see how running out of gas could do any damage to the cats. Go reread the manual and let us know what it says.
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  #14  
Old 09-24-2012, 02:07 PM
OntheRock OntheRock is offline
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sure....did refill and had diagnostics run at several places (including a dealer), each confirming it was the Cats. My Indy cleared the error a few times...car runs for about twenty miles then it comes back on (again with the car seemingly running very well). My Indy is/was very perplexed that it's the cats too (he makes the same observation about them rarely failing). So I have more or less resigned myself to do it at some point, but it certainly doesn't seem to be an emergency.

The manual makes two references and I somewhat misquoted...on pg. 25 "Refill early to avoid damaging the catalytic converter; never attempt to drive to the last drop of fuel in the tank." Then on page 81 "Please refuel early, since driving to the last drop of fuel can result in damage to the engine and/or catalytic converter."

Funny side story...the dealer wanted $5,500 to repair and advised that at that price the car was essentially totaled...he offered to take it off my hands and work with me on a deal for a new/used car...suffice to say I declined.
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  #15  
Old 09-25-2012, 12:21 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by 540 M-Sport View Post
anyone with a proper scan tool that can graph the O2 sensor outputs
May I ask if these tools will do what you say above?
- The most often recommended BMW diagnostic tools & cable interfaces (1)

Quote:
Originally Posted by OntheRock View Post
The manual makes two references and I somewhat misquoted...on pg. 25 "Refill early to avoid damaging the catalytic converter; never attempt to drive to the last drop of fuel in the tank." Then on page 81 "Please refuel early, since driving to the last drop of fuel can result in damage to the engine and/or catalytic converter."
I've run out of gas more than a dozen times so if that's true, my cats must be toast by now.

I didn't know about that warning, so I'll cross reference it to this thread:
- How much gas should be left to cool the fuel pump (1)

PS: For some inexplicable reason, I haven't run out of gas lately though ...

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Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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  #16  
Old 09-25-2012, 07:08 PM
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Fudman Fudman is offline
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Quote:
Originally Posted by OntheRock View Post
The manual makes two references and I somewhat misquoted...on pg. 25 "Refill early to avoid damaging the catalytic converter; never attempt to drive to the last drop of fuel in the tank." Then on page 81 "Please refuel early, since driving to the last drop of fuel can result in damage to the engine and/or catalytic converter."
Running on empty can damage the cats but will not always damage the cats. The damage can be caused my a misfiring engine. This is from Bobistheoilguy forum:

"when the tank’s fuel level drops below 1½ gallons, the (fuel pump) sub-chamber is intended to provide a reserve for the fuel pump during vehicle cornering. However, the one-way valve may inadvertently act in such a way as to prevent fuel from refilling the sub-chamber, failing to provide the intended reserve. If this were to happen, the fuel pump could momentarily draw air causing the engine to experience a sporadic misfire condition resulting in possible damage to the vehicle’s catalytic converter"

I believe that this is a very rare occurance.
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  #17  
Old 09-25-2012, 07:46 PM
OntheRock OntheRock is offline
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Shocker I am the beneficiary of this rare occurrence ... Say, versus winning the lottery :-)
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  #18  
Old 09-26-2012, 04:44 AM
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bluebee bluebee is offline
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While I don't doubt what has been stated, may I ask what mechanism will damage the cats?

Seems to me, musing at the PC, if the fuel pump were to starve, then the fuel pressure would be low, right?

Would that then result in a lean condition misfire?

I had lean condition misfires for more than a year (SES lit but not blinking SES though) before I found the root cause in a broken CCV lower vent hose.

Lean conditions happen all the time. I can see that causing high heat in the cylinders, which, I guess, can damage the cats, but the cats operate at high temperatures to start with.

So, may I ask, while a rich condition can 'clog' the cats ... what is the mechanism of damage to the cats due to the fuel pump starving?
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Each repair should invariably add to our knowledge base by the process of inexorable incrementalism.
Your job, in return, is to read the suggested threads, where the best people will always add value to those threads, either by pictures or by descriptions, so the next person with the same problem stands on your shoulders.
See also: E39 Bestlinks & How to easily find what you need
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  #19  
Old 09-26-2012, 05:01 AM
pshovest pshovest is offline
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This strikes me as possible, but very unusual. After all there is a code for "cyl x mis-fire w/fuel cutoff", which has the purpose of preventing cat damage.
How many miles on cats?
Can you post all the codes you've got?


Quote:
Originally Posted by OntheRock View Post
Shocker I am the beneficiary of this rare occurrence ... Say, versus winning the lottery :-)

Last edited by pshovest; 09-26-2012 at 05:20 AM.
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  #20  
Old 09-26-2012, 05:18 AM
pshovest pshovest is offline
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Not all misfires are the same. A faulty coil for example allows an unburnt air-fuel mixture to enter the cat. The O2 sensor sees the air as a lean condition and DME adds more fuel. Allowing an unburnt air-fuel mixture to enter the cat can greatly increase cat temp causing failure or meltdown. This is why there is a code for "cyl x mis-fire w/fuel cutoff", which has the purpose of preventing cat damage.

I agree that it's difficult to believe this could happen after running out of gas, in view of the misfire handling features in the DME.



Quote:
Originally Posted by bluebee View Post
While I don't doubt what has been stated, may I ask what mechanism will damage the cats?

Seems to me, musing at the PC, if the fuel pump were to starve, then the fuel pressure would be low, right?

Would that then result in a lean condition misfire?

I had lean condition misfires for more than a year (SES lit but not blinking SES though) before I found the root cause in a broken CCV lower vent hose.

Lean conditions happen all the time. I can see that causing high heat in the cylinders, which, I guess, can damage the cats, but the cats operate at high temperatures to start with.

So, may I ask, while a rich condition can 'clog' the cats ... what is the mechanism of damage to the cats due to the fuel pump starving?

Last edited by pshovest; 09-26-2012 at 05:19 AM.
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