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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 03-17-2013, 11:39 AM
Petlover Petlover is offline
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Oxygen Sensors Replacement

If I am correct, there are two Upstream O2 Sensors and two Downstream O2 Sensors, are they supposed to replaced every 100K miles. My car is up to 99.5K already.
Does it have any signs that they need to be replaced? or just wait until the service light shows up on the display.
These sensors are not cheap at all, it runs about $60 a pc

Are they supposed to be replaced all at the same time also?

T.
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  #2  
Old 03-17-2013, 01:19 PM
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The post-cat sensors monitor cat performance. One usually changes them as a test to see if the cats really are going bad.

The pre-cat sensors tend to get lazy at about that mileage. If you have noted a drop in fuel mileage, might be a good idea to replace them. There can be several different codes that point to the sensors.

If the car is running okay, don't futz with it.
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  #3  
Old 03-17-2013, 10:10 PM
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These threads, found by typing /oxygen sensor F3 in the best links, might help:
- 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)

See also:
- BMW_E39_Oxygen_Sensor_And_Fuel Trim.pdf (325.7 KB, 449 views)
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See also: E39 Bestlinks & How to easily find what you need
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  #4  
Old 03-18-2013, 06:24 PM
occhis occhis is offline
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I do the pre-cats every 100k. Just did the second round this past fall.
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  #5  
Old 03-18-2013, 06:47 PM
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I changed my pre-cats at 105K and noticed a 5% improvement in gas mileage. I did nothing to the post cats. I had no symptoms or CEL. I recommend replacing them.
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  #6  
Old 03-18-2013, 06:53 PM
daytradeoil daytradeoil is offline
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If you have the funds replace with bosch

easier on the wallet forking over the cash now opposed to down the line when you are searching for a cure to a problem...you at least will have one thing outta the way and off the list of possible culprits

Last edited by daytradeoil; 03-18-2013 at 06:54 PM.
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  #7  
Old 08-01-2013, 06:45 PM
wiredawg100 wiredawg100 is offline
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Oxygen Sensor Clarifications

Quote:
Originally Posted by bluebee View Post
These threads, found by typing /oxygen sensor F3 in the best links, might help:
- 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)

See also:
- BMW_E39_Oxygen_Sensor_And_Fuel Trim.pdf (325.7 KB, 449 views)
I'm thinking about replacing all my oxygen sensors; (2001 525i w/165k miles/M-54 engine type)
Getting the ses code referencing the O2 sensors/running lean that sort of thing. I had one O2 sensor replaced, "bank 2 front oxygen sensor" (what's on my invoice). Though, not exactly sure the location of this particular sensor. Meaning, I know it's one of the O2 sensors as shown in your diagram (pre-catalysts). I just don't know which one. At this point, a visual inspection they look practically the same and this was back in Nov, 2008/105K miles. As far as I know no other O2 sensors have been replaced.

So, with the mileage of the vehicle I was gonna simply replace all 4 O2 sensors, including the one replaced at 105k.

I've found a decent youtube vid that demonstrates the pre-catalysts:



Easy enough with the proper tools.

But, I have not been able to locate a decent vid for the O2 sensors (post-catalysts) underneath the vehicle. I'm a very visual type of guy. I comprehend better when I can see it...though, it sounds the post-cat O2 sensors are pretty straight forward, I just hope I can get them off.

If I can get some help with some clarifications:
In regards to the O2 sensor:
Bank 1 O2 sensor is located closest to fan?
Bank 2 O2 sensor is located closest to firewall?
(with that said, the O2 sensor I mention above that was replaced, should be the one nearest the firewall?)

I found O2 sensors on rockauto:
BOSCH Part # 13477 OE Type - Exact Fit; Heated; Wires: 4; Length (in): 12.8 One of our most popular parts Upstream; Engine Code M54 $50.79

BOSCH Part # 15109 OE Type - Exact Fit; Heated; Wires: 4; Length (in): 38.6 One of our most popular parts Downstream; Engine Code M54 $62.79

Upstream> is the sensors on top (pre-cat), engine compartment area as represented in your pic?

Downstream> is underneath the vehicle (post-cat) somewhere after the catalytic converter?

I appreciate any all help, thx.

1-Sept-2013:
Today, I replaced my precat O2 sensors Bank 1 and 2, not to bad. I want to tackle my post next weekend.
My question is: does the post and pre-cat O2 sensors connect to one another? And if so, I have to snake the wiring from the catalytic (below the car) to where the pre-cat O2 sensors are located?

Last edited by wiredawg100; 09-01-2013 at 08:34 PM. Reason: Adding remark & question
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  #8  
Old 08-01-2013, 06:59 PM
JimLev JimLev is offline
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Right, bank one is closest to the fan.
Pre CAT sensors are the ones closest to the engine.
Post are under the car.
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  #9  
Old 08-01-2013, 07:14 PM
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540 M-Sport 540 M-Sport is offline
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Quote:
Originally Posted by edjack View Post
The post-cat sensors monitor cat performance. One usually changes them as a test to see if the cats really are going bad.

The pre-cat sensors tend to get lazy at about that mileage. If you have noted a drop in fuel mileage, might be a good idea to replace them. There can be several different codes that point to the sensors.

If the car is running okay, don't futz with it.
+1. Unless there is a mileage drop, you can simply wait until one fails, then replace them both. You will get the SES light when one is way out of wack or the heater in it fails. They can and do get "lazy" in responsiveness which can affect gas mileage. I'm with Edjack, if all is well, leave them alone. Oh and $60 each is cheap, especially if those are Bosch...which is the only brand you should use, for best performance and least chance of issues. Have you ever purchase O2 sensors before? They can range up to $300 or more each....seriously.
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  #10  
Old 08-01-2013, 08:25 PM
wiredawg100 wiredawg100 is offline
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Thanks.
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  #11  
Old 08-02-2013, 05:22 AM
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Fudman Fudman is offline
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Quote:
Originally Posted by 540 M-Sport View Post
Oh and $60 each is cheap, especially if those are Bosch...which is the only brand you should use, for best performance and least chance of issues.
+1, stay with exactly what you have now (Bosch) to avoid possible issues. On my daughter's Honda, I replaced the OEM Denso O2 sensors with Bosch, thinking I was upgrading. Then I kept getting a check engine light. When I threw the old Densos back in, the problem went away. After replacing with new Denso O2 sensors, the problem never came back. While this may or may not happen with an e39, it is a lesson learned regarding O2 sensors and OBC sensitivity.
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  #12  
Old 08-02-2013, 05:41 AM
pshovest pshovest is offline
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Post your codes. Lean codes are far more likely to be vacuum leaks then O2 sensors.
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  #13  
Old 08-02-2013, 07:29 AM
wiredawg100 wiredawg100 is offline
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delete

Last edited by wiredawg100; 08-02-2013 at 07:33 AM.
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  #14  
Old 08-03-2013, 03:38 AM
tcbDE tcbDE is offline
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Quote:
Originally Posted by edjack View Post
The post-cat sensors monitor cat performance. One usually changes them as a test to see if the cats really are going bad.

The pre-cat sensors tend to get lazy at about that mileage. If you have noted a drop in fuel mileage, might be a good idea to replace them. There can be several different codes that point to the sensors.

If the car is running okay, don't futz with it.
Quote:
Originally Posted by 540 M-Sport View Post
+1. Unless there is a mileage drop, you can simply wait until one fails, then replace them both. You will get the SES light when one is way out of wack or the heater in it fails. They can and do get "lazy" in responsiveness which can affect gas mileage. I'm with Edjack, if all is well, leave them alone. Oh and $60 each is cheap, especially if those are Bosch...which is the only brand you should use, for best performance and least chance of issues. Have you ever purchase O2 sensors before? They can range up to $300 or more each....seriously.
All oxygen sensors are subjected to aging effects, basically slower response and greater inaccuracy. The technical customer information from Bosch lists the design lifetime as 100k mi, and with good reason. In my experience as an mech. engineer working on prototype and series production engines, we notice a very strong difference between new and "aged" O2 sensors. And we change them quite often.

Waiting until one fails is bad for many reasons. One, it may never "fail", even though it is giving inaccurate readings. Two, with aged sensors, the chance of having inconsistencies in lambda between cylinders much is greater. This obviously leads to not only decreased performance and increased consumption, but potentially dangerous lean conditions in some cylinders. Three, with a poorly-regulated lambda value, there are greater emissions and potential damage to the cat. Even if you "don't care about emissions," you are not gaining anything by increasing your emissions.

Quote:
Originally Posted by Fudman View Post
I changed my pre-cats at 105K and noticed a 5% improvement in gas mileage. I did nothing to the post cats. I had no symptoms or CEL. I recommend replacing them.
+1

I also replaced mine at 100k miles and noticed a slight improvement in responsiveness and gas mileage.
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  #15  
Old 08-03-2013, 06:41 AM
pshovest pshovest is offline
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A couple of points....
It's not at all clear that OP's lean codes are O2 sensors. Replacing sensors, hoping to fix lean codes, before ruling out vacuum leaks or other causes, is simply bad diagnostics.

Sure sensors get lazy and fail, but there are 24 codes specific to O2 sensor issues, including slow response codes. These codes are intended to prevent excessive emissions.

Gas mileage improvement and "slight improvement in response" claims are simply not believable. I record all gas purchases on several vehicles. Tank to tank MPG variation often exceeds 5% so claiming 5% improvement is VERY difficult to believe, as it is impossible for those making the claim to demonstrate with data.

If there was a basis for changing O2 sensors for fuel mileage improvement before sensor codes are set, why don't we see mfrs dyno data showing improvement in BSFC, Brake Specific Fuel Consumption?


ECU: MS43, Engine: M54, from 01.04.00 till 31.08.01
PCode BMW-FC PCode text
P0131 150 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 1)
P0132 150 O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
P0133 229 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
P0133 231 O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
P0134 150 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 1)
P0135 188 O2 Sensor Heater Circuit (Bank 1 Sensor 1)
P0137 152 O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)
P0138 152 O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0139 215 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
P0139 248 O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)
P0140 152 O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)
P0141 190 O2 Sensor Heater Circuit (Bank 1 Sensor 2)
P0151 151 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 1)
P0152 151 O2 Sensor Circuit High Voltage (Bank 2 Sensor 1)
P0153 230 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)
P0153 232 O2 Sensor Circuit Slow Response (Bank 2 Sensor 1)
P0154 151 O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 1)
P0155 189 O2 Sensor Heater Circuit (Bank 2 Sensor 1)
P0157 153 O2 Sensor Circuit Low Voltage (Bank 2 Sensor 2)
P0158 153 O2 Sensor Circuit High Voltage (Bank 2 Sensor 2)
P0159 216 O2 Sensor Circuit Slow Response (Bank 2 Sensor 2)
P0159 249 O2 Sensor Circuit Slow Response (Bank 2 Sensor 2)
P0160 153 O2 Sensor Circuit No Activity Detected (Bank 2 Sensor 2)
P0161 191 O2 Sensor Heater Circuit (Bank 2 Sensor 2)




Quote:
Originally Posted by tcbDE View Post
......All oxygen sensors are subjected to aging effects, basically slower response and greater inaccuracy. The technical customer information from Bosch lists the design lifetime as 100k mi, and with good reason. In my experience as an mech. engineer working on prototype and series production engines, we notice a very strong difference between new and "aged" O2 sensors. And we change them quite often.

Waiting until one fails is bad for many reasons. One, it may never "fail", even though it is giving inaccurate readings. Two, with aged sensors, the chance of having inconsistencies in lambda between cylinders much is greater. This obviously leads to not only decreased performance and increased consumption, but potentially dangerous lean conditions in some cylinders. Three, with a poorly-regulated lambda value, there are greater emissions and potential damage to the cat. Even if you "don't care about emissions," you are not gaining anything by increasing your emissions.


+1

I also replaced mine at 100k miles and noticed a slight improvement in responsiveness and gas mileage.
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Old 08-03-2013, 08:57 AM
tcbDE tcbDE is offline
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I realize that I might have been unclear, so I will try to restate my point / address your issues below.
Quote:
Originally Posted by pshovest View Post
A couple of points....
It's not at all clear that OP's lean codes are O2 sensors. Replacing sensors, hoping to fix lean codes, before ruling out vacuum leaks or other causes, is simply bad diagnostics.
I was not suggesting replacing a sensor in order to deal with a code. I was simply emphasizing the need/good practice of changing O2 sensors at a set manufacturer-specified interval. As in the sensor manufacturer. I assume you also change your oil at or sooner than recommended by BMW or the oil producer. Or your tires before they show steel belts, cracks or burst. In theory, you could continue to drive a vehicle in such a condition, but the performance would be degraded.
Quote:
Originally Posted by pshovest View Post
Sure sensors get lazy and fail, but there are 24 codes specific to O2 sensor issues, including slow response codes. These codes are intended to prevent excessive emissions.
That is a little mis-representative. There are six codes - high/low voltage (failure rich/lean), two for slow response, one for no activity, and one for the heater (for cold-start) - for four locations. Just because a sensor is within its hard limits does not indicate that it is performing optimally or "as good as new." No one would expect/assume that a sensor - which one day did not set a code and the next day did - SUDDENLY went from an excellent condition to a terrible condition. There is gradual degradation over time. Which is also why you may also not notice sudden or noticeable a drop in your fuel economy. Only if you compare fuel consumption to similar driving conditions when the engine was nearly new would a clear change over time be visible. Of course, if a sensor does fail, you will see a big change in performance.

At work, we use both multiple in-vehicle lambda sensors as well as more-precise test sensors, which clearly indicate sensor aging in as little as 30k km. When we change the O2 sensors, we see a significant improvement (as compared to the multiple-thousand dollar equipment we trust), despite not setting a code. Granted, the test conditions are harsher than a typical customer's vehicle would see, but the principle that new sensors perform significantly better cannot be disputed. Which is my point.
Quote:
Originally Posted by pshovest View Post
Gas mileage improvement and "slight improvement in response" claims are simply not believable. I record all gas purchases on several vehicles. Tank to tank MPG variation often exceeds 5% so claiming 5% improvement is VERY difficult to believe, as it is impossible for those making the claim to demonstrate with data.
I can understand your hesitation to believe my claim of increase in throttle response, as that is a subjective impression. However, fuel usage is another story. I'm sorry I did not explicitly state it: I noticed a minor but significant improve in multiple trials over the same section of road in similar conditions. With no other variables being changed. I am an engineer; I do this type of analysis for a living. And, yes, I record all my fuel consumption, too. We see this at work, too, on standardized tests like the NEDC, FTP, etc.
Quote:
Originally Posted by pshovest View Post
If there was a basis for changing O2 sensors for fuel mileage improvement before sensor codes are set, why don't we see mfrs dyno data showing improvement in BSFC, Brake Specific Fuel Consumption?
Are you asking me for this, or was it more of a rhetorical question? Do you see any maps of BSFC, let alone any "manufacturer's data" other full-load power and torque curves? If so, do share...

To the OP, change them all at the same time, as they are are all aged similarly.

Last edited by tcbDE; 08-03-2013 at 09:06 AM.
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  #17  
Old 08-04-2013, 06:12 AM
pshovest pshovest is offline
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Quote:
Originally Posted by tcbDE View Post
I realize that I might have been unclear, so I will try to restate my point / address your issues below.

I was not suggesting replacing a sensor in order to deal with a code. I was simply emphasizing the need/good practice of changing O2 sensors at a set manufacturer-specified interval. As in the sensor manufacturer. I assume you also change your oil at or sooner than recommended by BMW or the oil producer. Or your tires before they show steel belts, cracks or burst. In theory, you could continue to drive a vehicle in such a condition, but the performance would be degraded.
This is a thread about codes, so 8/1/13 poster, this is an old thread recently ressurected, is likely to believe that you think replacing O2 sensors will fix his codes. Oil analysis and tire inspection take the guess work out of when to change oil and replace tires. I'm simply proposing to use the mfr supplied tools ......OBD O2 sensor performance codes.....to decide when to change O2 sensors.


Quote:
Originally Posted by tcbDE View Post
.......That is a little mis-representative. There are six codes - high/low voltage (failure rich/lean), two for slow response, one for no activity, and one for the heater (for cold-start) - for four locations. Just because a sensor is within its hard limits does not indicate that it is performing optimally or "as good as new." No one would expect/assume that a sensor - which one day did not set a code and the next day did - SUDDENLY went from an excellent condition to a terrible condition. There is gradual degradation over time. Which is also why you may also not notice sudden or noticeable a drop in your fuel economy. Only if you compare fuel consumption to similar driving conditions when the engine was nearly new would a clear change over time be visible. Of course, if a sensor does fail, you will see a big change in performance........
Not at all mis-representative.....every sensor is monitored for (6) functions. BMW wouldn't monitor these parameters if they didn't impact emissions. The point is there are sensor performance codes. A sensor doesn't have to perform as good as new to perform acceptably. In many states in the US, the lack of sensor codes and resulting CEL/SES means vehicle is emissions compliant.....no direct exhaust sampling is made. If sensor performance has deteriorated to the point where emissions performance is impacted, why aren't these codes being triggered? Are the thresholds unrealistic, which would seem to violate the principles behind OBD2 monitoring, or is sensor performance still acceptable and the fuel mileage claims are bogus.

Quote:
Originally Posted by tcbDE View Post
......At work, we use both multiple in-vehicle lambda sensors as well as more-precise test sensors, which clearly indicate sensor aging in as little as 30k km. When we change the O2 sensors, we see a significant improvement (as compared to the multiple-thousand dollar equipment we trust), despite not setting a code. Granted, the test conditions are harsher than a typical customer's vehicle would see, but the principle that new sensors perform significantly better cannot be disputed. Which is my point.........
Sensor aging is a fact, but where's the evidence that sensor aging that doesn't trigger codes results in higher fuel consumption? If you've got data please post.

Quote:
Originally Posted by tcbDE View Post
.......I can understand your hesitation to believe my claim of increase in throttle response, as that is a subjective impression. However, fuel usage is another story. I'm sorry I did not explicitly state it: I noticed a minor but significant improve in multiple trials over the same section of road in similar conditions. With no other variables being changed. I am an engineer; I do this type of analysis for a living. And, yes, I record all my fuel consumption, too. We see this at work, too, on standardized tests like the NEDC, FTP, etc........
You are hardly the only engineer that participates in these boards.
With your technical background and all the sophisticated, high priced test equipment at your disposal I was hoping for something better then "I noticed a minor but significant improve in multiple trials over the same section of road in similar conditions." This is no different then the MPG improvement claims made by the other well intentioned contributors to these boards. If replacing every 100k mile sensor would result in a 5% improvement in gas mileage, the Sierra club, the EPA, and sensor manufacturers, through public service messages would be making these claims backed up with peer-reviewed data. The fact that they do not, speaks volumes.

Quote:
Originally Posted by tcbDE View Post
........Are you asking me for this, or was it more of a rhetorical question? Do you see any maps of BSFC, let alone any "manufacturer's data" other full-load power and torque curves? If so, do share.........
You've got access to test equipment that most of us don't. I was hoping you had some actual data showing fuel consumption improvement.

Quote:
Originally Posted by tcbDE View Post
.......To the OP, change them all at the same time, as they are are all aged similarly.....
BMW documentation says they use post cat sensors on some engines to make fuel trim adjustments but nobody replaces post cat sensors unless they have cat efficiency codes. Most who do replace them end up replacing their cats, because the O2 sensors weren't the problem. Advising someone to replace these without having codes strikes me as a complete waste of money. My 48k pre-cat sensors seem to work fine with my 156k post-cat sensors.

Last edited by pshovest; 08-04-2013 at 06:14 AM. Reason: typos
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  #18  
Old 08-04-2013, 01:52 PM
tcbDE tcbDE is offline
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I'm stepping out of the discussion after this post. I never meant for my post/this thread to turn into such a squabble. I was simply posting my experience/what I have gleaned from other engineers and manufacturers (some of it verbally, hence the lack of data.) Both of us have good points, it just comes down to what set of "beliefs" you adhere to. I look forward to responses nonetheless.
Quote:
Originally Posted by pshovest View Post
This is a thread about codes, so 8/1/13 poster, this is an old thread recently ressurected, is likely to believe that you think replacing O2 sensors will fix his codes. Oil analysis and tire inspection take the guess work out of when to change oil and replace tires. I'm simply proposing to use the mfr supplied tools ......OBD O2 sensor performance codes.....to decide when to change O2 sensors.
You are right - I missed that this was a thread resurrection. However, I still believe/ adhere to the advice I originally posted, namely that it is good practice to change O2 sensors at 100k. And, no, I disagree that OBD is the end-all-be all for testing sensors. A functions test of the sensors themselves (with synthetic rich or lean exhaust gas) would be comparable to oil analysis. OBD would be more of a "colored bars" approach to changing oil. Both are legitimate. I just believe - due to my experience - that one errs more on the side of non-degraded performance. Again, it is my belief, one that is backed up by the sensor manufacturer. See the Bosch website (or NGK or whoever) - the design lifetimes for all modern O2 sensors are 100k mi / 160k km (or 10 years), after which the performance has become degraded to the point that, statistically, the sensor error is not acceptable. Bosch even quotes up to a 15% fuel economy penalty on their website. Here is an example TCI. It is for a wideband sensor, and E39s use step-change sensors, but the majority of the relavant info is the same. I was unable to find a TCI for our sensors.

I am also currently writing a paper on a lambda-sensor replacement model, so I've had to do a lot of research into the topic. Which is why I have read a lot on the subject.


Quote:
Originally Posted by pshovest View Post
Not at all mis-representative.....every sensor is monitored for (6) functions. BMW wouldn't monitor these parameters if they didn't impact emissions. The point is there are sensor performance codes. A sensor doesn't have to perform as good as new to perform acceptably. In many states in the US, the lack of sensor codes and resulting CEL/SES means vehicle is emissions compliant.....no direct exhaust sampling is made. If sensor performance has deteriorated to the point where emissions performance is impacted, why aren't these codes being triggered? Are the thresholds unrealistic, which would seem to violate the principles behind OBD2 monitoring, or is sensor performance still acceptable and the fuel mileage claims are bogus.
We notice sensor degradation without tripping codes. I also didn't get a code when my thermostat or DISA valve failed.

All of the emissions rules are clearly stated, but if a manufacturer is still within the limit even with 10% error, that would be acceptable. Say, in this case, the new system performance was excellent relative to the standard; a decrease in aftertreatment performance would simply drop it to "good" or "sufficient," but the emissions still low enough for it to pass.

Quote:
Originally Posted by pshovest View Post
Sensor aging is a fact, but where's the evidence that sensor aging that doesn't trigger codes results in higher fuel consumption? If you've got data please post...

You've got access to test equipment that most of us don't. I was hoping you had some actual data showing fuel consumption improvement.
I cannot give out our data from work, as we have strict confidentiality rules. Sorry. It seems like an easy out. But posting on a forum is not worth my job. Some things you have to take on faith


Quote:
Originally Posted by pshovest View Post
You are hardly the only engineer that participates in these boards.
With your technical background and all the sophisticated, high priced test equipment at your disposal I was hoping for something better then "I noticed a minor but significant improve in multiple trials over the same section of road in similar conditions." This is no different then the MPG improvement claims made by the other well intentioned contributors to these boards. If replacing every 100k mile sensor would result in a 5% improvement in gas mileage, the Sierra club, the EPA, and sensor manufacturers, through public service messages would be making these claims backed up with peer-reviewed data. The fact that they do not, speaks volumes.
See that is just it. Not all sensors age exactly the same. And I do not believe that there are enough "real-world" studies to show this increase in fuel usage, be it 2% or 7% or whatever. That would be way too much variability. Which is why I defer to the manufacturer, Bosch, and their recommendation, based on controlled, reproducible data.

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Originally Posted by pshovest View Post
BMW documentation says they use post cat sensors on some engines to make fuel trim adjustments but nobody replaces post cat sensors unless they have cat efficiency codes. Most who do replace them end up replacing their cats, because the O2 sensors weren't the problem. Advising someone to replace these without having codes strikes me as a complete waste of money. My 48k pre-cat sensors seem to work fine with my 156k post-cat sensors.
"Seem to work fine" is about as strong as my "slight improvement" - not real strong. And of those that that also had to replace their cats, was an analysis performed as to the reasoning for the failure - a la your recommended oil analysis? I doubt it. And since when is following the majority a sure-fire way to do it correctly? Never. Hence my skepticism of blindly following everyone else. Granted, BMW enthusiasts are not "average" or quite like the rest of the driving population, but still. I mean, "everyone" used to change their oil at 3000 miles...

Again, we believe differently, which is fine. Each can read through our thoughts and choose for himself or herself.
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Old 08-05-2013, 05:15 PM
pshovest pshovest is offline
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Squabble? This is technical discussion....something many engage in to gain an understanding of complex issues. Thanks for the comments and my apologies if you were offended.
BTW, "seem to work fine" in the context I used it means "OBDII diagnostic say it's OK, so there's no reason to change it".
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Old 09-01-2013, 08:39 PM
wiredawg100 wiredawg100 is offline
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Pre & Post O2 Sensors Connect to One Another

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Originally Posted by JimLev View Post
Right, bank one is closest to the fan.
Pre CAT sensors are the ones closest to the engine.
Post are under the car.
Does the post and pre-cat O2 sensors connect to one another? And if so, I have to snake the wiring from the catalytic convertor(below the car) to where the pre-cat O2 sensors are located (engine block area)?
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Old 09-02-2013, 12:00 AM
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540 M-Sport 540 M-Sport is offline
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Originally Posted by wiredawg100 View Post
Does the post and pre-cat O2 sensors connect to one another? And if so, I have to snake the wiring from the catalytic convertor(below the car) to where the pre-cat O2 sensors are located (engine block area)?
No
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Old 09-02-2013, 01:12 AM
wiredawg100 wiredawg100 is offline
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Originally Posted by wiredawg100 View Post
Does the post and pre-cat O2 sensors connect to one another? And if so, I have to snake the wiring from the catalytic convertor(below the car) to where the pre-cat O2 sensors are located (engine block area)?
Quote:
Originally Posted by 540 M-Sport View Post
No
Okay, so can you tell me where the post-cat plug goes. I can't tell where mine goes. It just disappears up into the engine area from the catalytic converter. Pics would be very helpful, it's for a 2001/525i(E-39). I've seen others but it doesn't look like mine...I simply do not no where to look for the plug-in for the post-cat side.
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Old 09-02-2013, 02:59 AM
Burning2nd Burning2nd is offline
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are they supposed to replaced every 100K miles


Where did this digital blaspheme come from?
Where did you hear this crazyness?

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Old 09-02-2013, 05:46 AM
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Look under the plastic covers on fuel rail and valve cover. Change them one at a time so you don't mix up the connectors. Very unusual to need to change the post cat sensors. Why are you doing this?

Quote:
Originally Posted by wiredawg100 View Post
Okay, so can you tell me where the post-cat plug goes. I can't tell where mine goes. It just disappears up into the engine area from the catalytic converter. Pics would be very helpful, it's for a 2001/525i(E-39). I've seen others but it doesn't look like mine...I simply do not no where to look for the plug-in for the post-cat side.
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  #25  
Old 09-02-2013, 11:55 AM
wiredawg100 wiredawg100 is offline
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Quote:
Originally Posted by pshovest View Post
Look under the plastic covers on fuel rail and valve cover. Change them one at a time so you don't mix up the connectors. Very unusual to need to change the post cat sensors. Why are you doing this?
I was getting the running lean codes bank 1 & 2 (171 & 174). The pre-cat/Bank 2 was replaced awhile ago (about 105kmiles) but the others have not. I'm at about 166K miles, now. I changed out both the pre-cats yesterday, just looking the change out the post-cat, since I already purchased them...
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