# 4D16: SCR Efficiency -> How many replaced front NOx to fix?



## lpcapital (Mar 1, 2007)

I'll ask the same question here, hoping to get some feedback. In general I read this is the case but considering how finicky the emissions system of these car is and how BMW Service Centers have the tendency of just throwing [owner's] money and parts at problems I wanted to get some "long term" feedback.

Did the oil change over the weekend and did the routine scan with the BMW software. Turns out about 100 miles ago the DDE stored a 4D16 code. I run through the procedure: it turn off the meetering unit, you drive 30 minutes, come back, plug car back in, and check NOx sensor readings. I guess what happens is that driving around burns off any urea/ammonia stored in the SCR, saturates it with NOx and basically no more is processed and the pre and post sensor should read the same... Kinda clever...

After that is says that the variance between the two reading is greater than 25% and the front sensor is too low and the procedure says to replace the front NOx sensor and recalibrate.

Reading around it seems lots of people who take it to the dealer for the same code say the same: that the dealer replaced the front sensor.

I wanted to see if anyone has gone through this and if the replacement of the front NOx sensor effectively fixed the issue or not. Thanks for any feedback!!!


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## rippjd (Apr 30, 2006)

I'm not sure this helps at all, but I was getting the fault "4BB2 NOx sensor before DeNOx cat., plausibility NOx". I tried all the suggestions in the Rheingold test routine, but none eliminated the fault until I tried the last one... replace front NOx sensor. Fortunately at the time you could still buy the less expensive 13-62-8-511-666 SENSOR for about $165 if the Dealer was giving you 20% off.

Pulling the clamps off the various locations around the transmission to the transfer case is a pain. I'm sure it took me at least 2 hours for the swap.
Photo shows new one on top. Good luck!


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## lpcapital (Mar 1, 2007)

Good to see that both sensor design work!!!

After doing some research it turns out that Continental makes these sensor for pretty much all diesel engines that use SCR for NOX reduction. Obviously the BMW one is the most expensive: the one for the GM Duramax is about 150 bucks.

If the code ever trigger the CEL and the sensor needs replacement, I'll just visit the various parts departments and look for one that has the same connector on the side of the processing unit and a similar length in wire.

Btw, I was reading around that is not that uncommon for the NOX sensors, particularly the pre SCR one to deplete with time. The way they work is that they actually measure oxygen, not NOX directly. There is a catalityc element inside that splits the NOX in N2 and O2. The O2 is then measured similarly to the way a wideband works and the system can deduct how much NOX would have generated the measured amount of O2. I guess with time the catalytic element reduces its ability to split NOX or get poisoned by sulfur. Since the pre SCR sensor is the one exposed to most NOX and sulfur it is the first one to go south...


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## rippjd (Apr 30, 2006)

I agree with your logic on the pre-NOx sensor going first...

Here's a few more pics for your research. I'm hoping the new sensor lasts another 70k miles.


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## sirbikes (Aug 17, 2012)

I just had the upstream one replaced at 98500mi, but the downstream was so far out of whack relative to the new one I had to have that one replaced as well. Plan on replacing them in pairs if you have a lot of miles on them. Are there more than two in the X5d? That's what I'm worried about.


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## ingenieur (Dec 26, 2006)

There are only 2 - what do you mean out of whack? Did the dealer perform the ISTA test to determine this?


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## sirbikes (Aug 17, 2012)

I can't find my paperwork. They replaced the pre-SCR sensor, then the car threw an out of range code for the post-SCR sensor, so that was replaced. Then no more codes. This is a complicated system I don't think my mechanic understands how it works. I certainly don't ... something to do with calculating the quantity of urea to inject into the SCR cat to minimize NOx. But how the DDE determines that a sensor is bad ... no idea. All I know is that once the sensor is out of range, it's done ... no way to zero or reset it, can't be tampered with -- someone said it is this way due to govt regs.



ingenieur said:


> There are only 2 - what do you mean out of whack? Did the dealer perform the ISTA test to determine this?


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## lpcapital (Mar 1, 2007)

I did run through the Rhengold procedure. After the testing (which require a 30 minutes drive while the DDE has deactivated the metering valve so no urea is injected) it said to replace the front sensor and then get back in Rheingold to calibrate the sensor. I run the calibration without replacing the sensor.

Now, like I said the code was pending and the frequency was 1. Since then nothing has changed, the frequency hasn't increased. Now that you say that I'm wondering if the calibration simply reset a certain baseline and I'm now polluting more.

To answer the question. One of the ways the DDE performs the testing is by temporary turning off the urea injection which stops the reaction in the SCR that breaks down the NOx. With the sensor off the DDE can determine if the sensors are within range by checking the pre and post SCR which should read the same (since no NOx is processed). I'm sure there's more plausibility tests, but this one is the one that Rhengold uses to determine if a sensor is out of range.

There is also a calibration function which is carried out after one sensor is replaced. It must be carried out to reset the baseline. This seems to contradict your statement that there is no way to reset it...


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## sirbikes (Aug 17, 2012)

What you say here makes sense ... if a new sensor is installed and this procedure is run to check the baseline readings, and they don't read the same, then the calibration should reset one or both to a new baseline. But I think what my mechanic was saying was that after they installed the new one, the old one could was too far out of range. Maybe BMW has a correction quantity limit? Maybe you cannot reset or zero these things. The sensor has it's own independent control unit that can't be tampered with. I guess it makes sense if you think of it from the EPAs perspective -- otherwise you could theoretically just zero out a bad sensor to pass an emissions check.



lpcapital said:


> To answer the question. One of the ways the DDE performs the testing is by temporary turning off the urea injection which stops the reaction in the SCR that breaks down the NOx. With the sensor off the DDE can determine if the sensors are within range by checking the pre and post SCR which should read the same (since no NOx is processed). I'm sure there's more plausibility tests, but this one is the one that Rhengold uses to determine if a sensor is out of range.
> 
> There is also a calibration function which is carried out after one sensor is replaced. It must be carried out to reset the baseline. This seems to contradict your statement that there is no way to reset it...


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## ingenieur (Dec 26, 2006)

As with the SCR catalyst, metering valve and DPF the DDE calculates an aging factor for the NOx sensors.

If the DDE is replaced it needs to know how many km are on these components along with the injector calibrations.

If any of these are replaced the SCR system adjustment service function in Rheingold (ISTA) should be run to reset or write in the "baseline" values. 

This is described in the repair instructions and functional description documents.


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## 335duff (Jul 30, 2015)

Hello, just got my car back from this service. My car threw code 4D16 and dealer replaced both NOx sensors at no cost to me. Car seems to run smoother now, and is getting much better mileage than before replacing the sensors.

Best of luck!


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## G335d (Apr 22, 2014)

Ditto here. I just picked up my car Wednesday after this same diagnostic process.

A week after my CBU cleaning at 81,000 miles I had another SES light come on, followed closely by a red warning message about incorrect def fluid. Working with the BMW Regional tech person, the dealer replaced the upstream sensor. This did not clear the code, so they then replaced the downstream sensor. BMW also picked up the rather expensive costs of this diagnosis/replacement, which was much appreciated since I picked up the cost of the CBU cleaning.


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## sirbikes (Aug 17, 2012)

Good thing it occurred right after the cleaning otherwise they might not have covered it. In my case, I was too far out of warranty, even well beyond the 8yr/80k federal emissions to have them cover it.



G335d said:


> Ditto here. I just picked up my car Wednesday after this same diagnostic process.
> 
> A week after my CBU cleaning at 81,000 miles I had another SES light come on, followed closely by a red warning message about incorrect def fluid. Working with the BMW Regional tech person, the dealer replaced the upstream sensor. This did not clear the code, so they then replaced the downstream sensor. BMW also picked up the rather expensive costs of this diagnosis/replacement, which was much appreciated since I picked up the cost of the CBU cleaning.


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## sirbikes (Aug 17, 2012)

Ok I had both NOx sensors replaced and things were fine for a few weeks. Then yesterday I got a P20EE code: SCR oxides of Nitrogen catalyst efficiency below threshold. See pic attached. I'm hoping it's not the scr cat or the metering valve now. How much are these to replace? I'm afraid my emissions systems are falling to pieces. After 100k miles, I guess it's had a good run.


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## serge1 (Jan 6, 2012)

2011 35d, 90,000 miles. Yesterday, check-engine light popped up. Read it with OBD tool today, it shows this error:
P20ee - SCR Oxides Of Nitrogen Catalyst Efficiency Below Threshold (Bank 1)

This is exactly the same message I got 2 years ago (at 49,960 miles). At that time both pre and post cat sensors were replaced (according to SA).

@lcapital, your theory may hold some water..


lpcapital said:


> Btw, I was reading around that is not that uncommon for the NOX sensors, particularly the pre SCR one to deplete with time


So, these diesels have an essentially a maintenance part that lasts roughly 50K miles and costs $2,000.

P.S. Vehicle is drivable without any countdowns to no start - that's good. In addition, after I cleared the code, the check engine light went away. But I am sure that doesn't mean much - it will likely be back.

P.P.S. Exploring my options now. Worst case, I can have it all gutted for 2K bucks and have more power at my disposal without having to worry about it again.


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## finnbmw (Jul 6, 2008)

On our 2010 X5d I've been getting the 4D16 fault with the yellow CEL, but I've been ignoring it because the CEL goes away after a while and the X5 runs normal. Yesterday a new fault popped up along with the CEL (from Carly):

_NOx sensor according DeNOx catalyst NOx plausibility average NOx concentration is not plausible to NOx signal upstream of the DeNOx catalyst during load changes 
Code: 46A4
_

Anybody have an idea what this could be? I am tempted to take my X5 back to the dealer to see if I could get a goodwill repair as both my NOx sensors were replaced 26,000 miles ago. They should last longer...


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## G335d (Apr 22, 2014)

When my dealer replaced my NOx sensors, I verified that the usual 2 year guarantee on those parts would apply, I would think that would be the case for you.


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## floydarogers (Oct 11, 2010)

serge1 said:


> ...
> So, these diesels have an essentially a maintenance part that lasts roughly 50K miles and costs $2,000.


The NOx sensor is just like any lambda probe (O2 sensor): they have a finite lifetime. Most O2 sensors last 100K +. According to a couple wikipedia or other sources I've seen, most NOx sensors have a 90K mile or so lifetime: remember, they've got to last at least 70K miles to pass California Emissions.

I suspect that mine are starting to go: 104K miles and I've got a 4d16 sitting in my DDE.


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## serge1 (Jan 6, 2012)

G335d said:


> When my dealer replaced my NOx sensors, I verified that the usual 2 year guarantee on those parts would apply, I would think that would be the case for you.


Called the dealer; yes there's a 2 year warranty indeed. Unfortunately, the sensor was replaced on Dec 20, 2013, so I am out of luck by a few weeks. Even if it was within, it would probably be a hassle to get it covered (as suggested by lcapital in another thread; I agree).



serge1 said:


> 2
> So, these diesels have an essentially a maintenance part that lasts roughly 50K miles and costs $2,000.


It looks like I grossly misrepresented the price of the sensors. That price may be the total repair cost, labor, parts, etc. Or maybe price for 2 sensors together.

The pre-cat sensor by itself is around $250 online.


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## serge1 (Jan 6, 2012)

finnbmw said:


> a goodwill repair as both my NOx sensors were replaced 26,000 miles ago. They should last longer...


I was told by an SA at local stealership that miles are unlimited, but time does apply. Has it been 2 years since?

You may end up still paying some for diagnostics despite warranty on the parts, especially if they find other issues (we all heard stories about throwing parts at the problem).


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## finnbmw (Jul 6, 2008)

Unfortunately, it's past two years by now. I have an appointment for next week to diagnose, will speak with the Service Mgr prior to see if any goodwill will apply to my case. Never hurts to ask


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## vadia (Jan 12, 2015)

*x5 35d*



serge1 said:


> Called the dealer; yes there's a 2 year warranty indeed. Unfortunately, the sensor was replaced on Dec 20, 2013, so I am out of luck by a few weeks. Even if it was within, it would probably be a hassle to get it covered (as suggested by lcapital in another thread; I agree).
> 
> It looks like I grossly misrepresented the price of the sensors. That price may be the total repair cost, labor, parts, etc. Or maybe price for 2 sensors together.
> 
> The pre-cat sensor by itself is around $250 online.


Where did you see pre cat NOX sensors for $250. BMW jack the prices 500-600$. BMW trying to make easy $$$$. They update the part and increase the price. from 200$ to 500$ Pre cat and post cat same price, Most likely the same sensors just different length of wires 915mm and 415mm. I just had mine 2011 x5 35D diagnose with NOX sensor bank 1, code p2201, BMW code 4976. Probably would end up replacing both of them.


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## sirbikes (Aug 17, 2012)

I may have two for sale (pre and post NOx sensors) that are nearly new (installed in Dec). I'll put an ad in the classifieds along with proof of purchase in a couple of weeks. I paid $500 ea. Sell for half OBO.



vadia said:


> Where did you see pre cat NOX sensors for $250. BMW jack the prices 500-600$. BMW trying to make easy $$$$. They update the part and increase the price. from 200$ to 500$ Pre cat and post cat same price, Most likely the same sensors just different length of wires 915mm and 415mm. I just had mine 2011 x5 35D diagnose with NOX sensor bank 1, code p2201, BMW code 4976. Probably would end up replacing both of them.


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## vadia (Jan 12, 2015)

*x5 35D*



sirbikes said:


> I may have two for sale (pre and post NOx sensors) that are nearly new (installed in Dec). I'll put an ad in the classifieds along with proof of purchase in a couple of weeks. I paid $500 ea. Sell for half OBO.


Interesting. I already bought mine 440$ from eBay. But I rather buy 2 
of them for same amount of money. Why are they off the car?


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## Vincentjwm (Sep 14, 2015)

Can anyone guide me through how to use Rheingold to deactivated the metering valve in DDE like Ipcapital to reset the front sensor? I couldn't find where I should perform this.


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## JTC1 (Oct 6, 2016)

I just had the P20EE code pop up for me yesterday. Folks with experience troubleshooting this, I have read in a few older posts that the sensors may not actually be bad, but that the urea injector may be clogged. Anyone else have this experience? My car sat for a month at the dealership before I bought it, so I am hoping that may be the case for me. Cleaning a clogged injector is much cheaper than replacing two sensors.


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## RPsX5d (Nov 2, 2010)

When you said "_sitting in my DDE_" . . . did you mean you can see it with a scanner (Carly?) but it has NOT tripped the CEL?

Second question - have you tried clearing it, charged up your battery and re-scanned it again? Did the 4D16 return and still no CEL?



floydarogers said:


> . . . I suspect that mine are starting to go: 104K miles and I've got a *4d16 sitting in my DDE*.


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## RPsX5d (Nov 2, 2010)

Ipcapital - when you refer to "front NOx sensor", you meant *upstream* or "pre" to the SCR catalyst - right?

Between the two, pre and post NOx sensors, upstream or pre NOx sensor is the one that is likely to go first - true?

I too have a 4D16 stored, but no CEL. Cleared it once, charged the battery (7 years old) overnight, cleared the fault code . . . ~1,000 miles or so later 4D16 came back. Beginning to suspect this 4D16 is real and not a transient case.

I have been chasing a 20% drop in fuel mileage for the past three months or so . . . fairly certain the O2 sensor and EGR high-pressure valve are functioning fine . . . the pattern they exhibit on the OBD reader is very very consistent.

EGR low-pressure valve (EPDW) is showing an erratic pattern . . . still watching it.

The plan is to replace the battery, fuel filter and a few other planned maintenance items like coolant, air filters (cabin & engine), etc in early summer . . . and see if fuel filter makes any difference to the reduced mpg before doing anything specific for the 4D16 condition.

Still have another 8,000 miles before the BMW 120K miles extended warranty for the DEF active tank is reached . . . my extended warranty also runs out at 120K miles.


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## Chill_X5 (Feb 1, 2017)

WOW I got another slap in a face P20EE code came on today. So I put about 3 gallons of DEF fluid reset CEL with regular OBD2 scanner. Unfortunately it did not work. its a part of historic codes but CEL light is still on.


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## RPsX5d (Nov 2, 2010)

I don't think a legit 4D16/P20EE can be cleared by adding DEF . . . if everything is working properly, low DEF will trigger an amber colored 999 message on the iDrive . . . .

Not sure how a *real* 4D16 fault code works . . . i.e. do they first get stored in the DDE for some time before it triggers a CEL?


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## Chill_X5 (Feb 1, 2017)

For the past 2 month car has been scanned often because it had issues with EGR Cooler that I end up replacing. No issues with DEF were found. From research i've done P20EE has 3 solutions, clean DEF nozzle, add or flush DEF and/or replace NOx sensor (the cost of replacement is a freak show). 
How many NOx sensors does X5 have? 
Are there any aftermarket once available?


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## RPsX5d (Nov 2, 2010)

Two NOx sensors . . . I extracted the attached image from a BMW Diesel Technology PDF, shows the location of these two NOx sensors.

Here's a thread you might find useful . . . #2 post talks about using an older version (much cheaper) of the NOx sensor . . . not sure if you can still get these . . .


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## Chill_X5 (Feb 1, 2017)

I checked online and with a dealer/stealer and the cheaper once are long gone. After 30-50 min drives I can smell Urea outside a car. Also I started to monitor engine temperature it was 17 F in Chicago this morning but car only got to 74-75 C.


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## Chill_X5 (Feb 1, 2017)

By any chance does anybody have part numbers?

I got 13628589846 and 13628589844 just dont know which is pre and post.


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## ingenieur (Dec 26, 2006)

Chill_X5 said:


> I checked online and with a dealer/stealer and the cheaper once are long gone. After 30-50 min drives I can smell Urea outside a car. Also I started to monitor engine temperature it was 17 F in Chicago this morning but car only got to 74-75 C.


What you smell is NH3 - ammonia which is a symptom when too much urea is dosed. Considering the age of your car and if the SCR parts are original the first thing is to perform the ISTA test for the DEF quality, metering valve and SCR efficiency function test.


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