Xentry Diagnostics Value 751 - Factor for adaption of component 'NOx sensor upstream of SCR catalytic converter'

[EdPhil]

Hi: This is my first post, so here's a bit of background on me and what I know first. Sorry it's a bit long-winded.

I have a late 2014 CLS 350 Bluetec (the facelift version) that I maintain myself as I have decent garage facilities, tools, and also Xentry diagnostics 2021. I have been having issues with NOx sensors for the past 18 months. Each original Mercedes NOx sensor comprises a Continental control unit and an NGK NS 11A probe connected together by eight wires. It's doubtful that both fail and need replacing at the same time, and it's relatively easy to cut the wires and replace one or the other - albeit that it's normally the probe that fails. It does not seem possible to source and purchase new NGK NS 11A probes (?), but third-party probes are available from various sources. Replacing a probe is far cheaper than buying a new complete sensor from Mercedes.

I am led to believe that just replacing the probes may not work if you have an old version control unit, which may malfunction if the Mercedes Diesel Software update has been applied to your car. Although there is only one part number for the two NOx sensors (one upstream of the catalytic converter and one downstream) fitted to my vehicle (A 000 905 34 03) there are numerous versions of the Continental control unit: My originals were 5WK9 6681C and version ZGS 002 Q 02, which worked fine for about 8 months with new third-party probes wired on and then one started playing-up. I then obtained a newer used (but working) complete sensor 5WK9 6681D and version ZGS 002 Q 05 (with NGK NS 11A probe), which did not seem to work properly with the original 5WK9 6681C and version ZGS 002 Q 02 one still fitted in the other position. Details obtained from a Mercedes forum led me to believe that versions 01 and 02 control units will not work with versions 03 and above. Subsequently, I ordered a couple of refurbished control units (Mercedes Continental control units with new third-party probes). I had expected them both to be 5WK9 6681D ZGS 002 Q 05, but one was a Q 03 and one a Q 04. The Q 04 was/is faulty, but the Q 03 seemed OK and I have that currently fitted in the upstream (front) position and the Q 05 version is now fitted in the downstream (back) position. These seem to be working correctly now as the monitoring in Xentry looks as I would expect - see attached image. However, there is still a generic fault evident with the front sensor.

Using Xentry diagnostics (just like they will do at the Mercedes dealership), there are just two tests for a NOx sensor using the adaptor cable W 651 589 0063: One is to test the voltage across pins 1 and 2, and the second is to test the resistance across pins 3 and 4. I have sensors that pass these tests fitted to the car. Note that the malfunctioning Q 04 control unit also passes these tests.

Once NOx sensors are changed they are 'learned' using Xentry. There is detail that coding is necessary, but I am led to believe that this is really just to update the Mercedes database about the new serial numbers of the control units fitted. Note that these serial numbers are part of the firmware in the units and not the numbers printed on the bottom of the control units. These numbers can be read in Xentry during interrogation of an active fault code and choosing the replace NOx sensor option.

Using Xentry diagnostics, you can also view individual actual values and I have a group for NOx as shown in the second image. AND THAT'S WHAT THIS POST IS REALLY ABOUT. The Actual Value 751 Factor for adaption of component 'NOx sensor upstream of SCR catalytic converter' is showing a value of 'Fault: Error details: Request out of range'. I currently have no idea where this actual value is derived from or how to reset it and would be obliged if someone could help. It may well be this 'adaption' value is causing the error to be generated for a perfectly serviceable sensor(?).

From my experience, it's really difficult to find any proper technical information on the actual control units, what the different versions are and what the software/firmware incompatibilities are. Mercedes seem particularly unhelpful.

Best Regards;

Edward

 

[ajlsl600]

On this issue I would reccomend going to one of the known indies on here, they will likely be well informed on this matter, Frankly all to do with nox sensors of which there are a number of varients (at least 3 I think)? Of which two were problematic(did not properly resolve matters) I believe, but stand to be corrected that the third with dreaded update has actually fixed near all issues (for now!!) problem I understand with updates is it seems all gets updated not just Yr nox sensors and there have been, if I read correctly, some other issues as a result of benz updates. And there have been others that have been happy with updates
I myself am of the view that until circumstances force me, I go no where near updates. If it ain't broke don't fix it. In anything to do with mb dealers
Good luck. Interesting post that.

 

[supernoodle]

"Request out of range' is standard ISO 14229 negative response code 0x31.

All that data you see on the Xentry screen is obtained by the tool sending a request to read a DID from the module.
The module has to give a response. Normally of course that is positive, so you'll just shown the data. When negative, it has to give a reason why.

Normally the tool wouldn't request data from a DID that didn't exist, unless the module or software on it wasn't what was expected.

https://automotive.softing.com/fileadmin/sof-files/pdf/de/ae/poster/UDS_Faltposter_softing2016.pdf

 

[EdPhil]

Members - thanks for the replies.

I agree with ajlsl600 about updates in general. Unfortunately, I took the car to Mercedes about 2.5 years ago to code in towing hitch electrics I had installed myself before I had the facilities to do the coding myself. I believe it cost me nearly £200 for about an hour connected to their system and they automatically installed the diesel software update as well (and probably other updates) without me knowing - I did get a 'free' cup of coffee but am a bit wiser now.

I also have a 2011 E Class 250 that I have had since 2012, and when I was working (I am now retired) it was maintained by Mercedes main dealers in Bristol, and Gloucester, but in later years I tended to use the independent PSC in Cheltenham, who have always given me great service at a much more reasonable price. I could just ask them, but even they will probably give the 'standard' 'book it in for a diagnostics check' response, and I was hoping to avoid that as I now have plenty of time, but not money.

Supernoodle is homing-in on my problem. Yes, I am fairly certain that detail is being requested by Xentry from a module, but which one? If I swap the NOx sensors over, the fault is still there for the upstream (front) sensor and the adaption values details as per the image in the original post above do not change (0 for the front and 1 for the rear), so the information is not coming from the NOx sensors(?). Other values do come from the sensors and they 'learn in' correctly using the reset learned values function in Xentry. There are no teach in processes for NOx sensors in Xentry. What I really need to know is exactly where the error data is being derived from and how to correct/reset it. If the NOx sensors can be swapped over but the fault still occurs on the front it's logically really just a software/firmware problem? I know the front sensor uses four wired connections and the rear five, but don't think that's the issue.

I have read many forum posts about NOx sensor problems, and the main conclusion I come away with is that even Mercedes dealers don't seem to fully understand the problems and just keep swapping out what may be perfectly good sensors for new ones until the problem goes away. Without access to proper technical information on the actual control units, what the different versions are and what the software/firmware incompatibilities are with the CDI control unit updates it's just a guessing game.

I would assume that the very latest versions of the Mercedes NOx sensors should work with a vehicle that has had all the CDI control unit updates applied and that's probably how Mercedes are getting the job done - at a price.

Anyway, for my purposes, the NOx sensors fitted appear to be working correctly to me now, but I am still getting the fault 'P2201E3 The NOx sensor 1 (cylinder bank 1) has a malfunction', which on its own is not very helpful. In Xentry, however, there is a full description of fault codes that does not include P2201E3, but does have P2201E1 details. This details that the fault is set when the upstream NOx sensor deviates from the characteristics map value. So I am assuming this is probably relevant to fault P2201E3 and am trying to determine where and what this characteristics map value is and if and how the Actual Value 751 factor error is (or is not) affecting this - any detailed information would be appreciated.

For further information I have attached images of the details of the current NOx sensors fitted, and also the detailed information relating to fault code P2201E1.

Best regards:

Edward

 

[Srdl]

interesting thread, Edward, even though I don’t understand much of it! There are two on here who might have the necessary knowledge so I’ll tag them in the hope they see it. Welcome to the forum!
@alexanderfoti @Uncle Benz

 

[ajlsl600]

U have the right guys now.. Its somewhat over complex but ruling out the actual sensor, probe will help pinpoint the actual issue . I suspect that an up to date control unit will be required as for sure benz won't be interested in anything but replacements and it may turn out time consuming for indies and they are, for reasons we understand, fairly busy, I wonder what reading the data from a functioning up to date nox sensor control unit, units might reveal?
Watching with interest.

 

[EdPhil]

Srdl and ajlsl600: Thanks for the replies.

Essentially, it's access to proper truly detailed technical data about how Mercedes' NOx systems work (hardware and software/firmware) and how these components and software/firmware have evolved over time that is required. I am sure that Mercedes have it, but are probably reluctant to release it as it will allow people like me to repair components or source suitable used units without being sucked in to just blind and costly new replacements that keep their marble showrooms solvent. Replacement NOx probes seem to be installed because of a fault code and its title without ever looking properly at the detailed reason the fault occurred. That's why so many people have the sensors replaced but then still get faults occurring.

Mercedes Xentry diagnostics, EPC and WIS all have lots of very good information that helps describe, fault find and service Mercedes vehicles, but I believe it's mostly at what I see as an intermediate technical level, with no real detail of the 'black boxes'' internals and the software/firmware. Consequently, as this is what the Mercedes technicians use, they will mostly diagnose a fault to that level and, as per ajlsl600's insight, tell you that new parts are required, because that's what the system is all about(?). I do have some sympathy with Mercedes though, as commercially it's not really viable to spend hours diagnosing a niggly problem to a very detailed level, and then spend time explaining exactly what the problem is and discussing various remedial options and a range of costs with the customer. Customers using the main dealers are obviously not too worried about costs anyway, otherwise they would be using the independants or DIY.

Let's hope that someone out there has some detail available.

Best Regards:

Edward.

 

[alexanderfoti]

Anyway, for my purposes, the NOx sensors fitted appear to be working correctly to me now,

How are you determining the above?

The "map value" it refers to, is not a single value but a comparision of load/temp etc thats parts of the fuelling map.

 

[ajlsl600]

Unless he gets hold of an ex mb auto electrician I doubt this will get very far, even then I doubt most will be trained to that level, dealers will. Most likely see fault and just be at replacement level what op wants is likely a issue for maker/designer of component.or someonecon here with a lot of electronic experience and an inquiring mind.
I wish him luck..

 

[supernoodle]

Yes, I am fairly certain that detail is being requested by Xentry from a module, but which one? If I swap the NOx sensors over, the fault is still there for the upstream (front) sensor and the adaption values details as per the image in the original post above do not change (0 for the front and 1 for the rear),

Are you swapping just the sensors or sensor and module?
The module would be the unit responding to the DID request.
If you really want to delve deeper, eg to see if the DID request is the same for the upstream and downstream you could use a CAN device to log the diagnostic traffic and watch for the negative response. The message would show the module IDs too.

With a three byte fault code, eg P2201E1, the first two P2201 detail the component or subsystem, the last is the failure type byte. So you'd be correct in assuming P2201E1 and P2201E3 world be similar. For example 85 is Signal above allowable range, 86 is Signal Invalid.
Never seen anything above 98, so not sure if these are MB specific or I've just got an old spec.

The plausability test I expect isn't a comparison to a single map but rather a modelled NOx value. Likely if the measured engine out NOx is significantly different to the modelled it assumes there is a sensor problem. Of course there could be a base engine issue that means engine out NOx really have changed.
Some of the first gen SCR systems were even worse(not MB). They only had a downstream NOx sensor. They would throw a fault on SCR efficiency being too low based on the fact the downstream NOx being too high. As the engine out was only modelled, the SCR might have been working fine, but as the fault said SCR efficiency, they were getting changed unnecessarily. Base engine NOx had increased but wasn't being measured.

NOx sensors can drift so an adaption value is learnt at least for the 0 value. This would normally happen at some time after engine has stopped and sufficient time has passed to ensure exhaust gases have cleared. This could be the value 751 here.

 

[EdPhil]

Thanks for replies:

'Superficially', I am taking the current NOx sensors fitted to be working correctly because of the graphing of their operation in a four minute test shown in the image attached to the initial post. That's how I would expect them to operate, and what I mean by that is the readings of the NOx levels in the exhaust at various engine speeds look OK to me. My understanding, basically, is that the front sensor reads the NOx level to trigger the AdBlue dosing and the rear sensor reads the level after the dosing, so it should be lower (I know there's a bit more to it than that). When the engine is in deceleration mode or there are throttle changes the measurements will change accordingly to adjust the dosing. The NOx reading of the rear sensor drops to near 0 when idling. AdBlue dosing also looks reasonable from other data. Note that I have graphing of sensors installed that are not working properly, and they look very 'messy' with the bad sensor readings easily recognisable on the graphing. Even non-operational bad sensors can give readings and they look very erratic compared to the graphing image above. If anybody has a graphing image of a brand new pair of sensors in operation, I would love to see it.

Note that I have found that to get the NOx sensors up to an 'Operational' state for the graphing testing under Xentry normally requires the vehicle to be taken for about a 20 miles run and then connected to Xentry without switching off the ignition and the engine still running. Sitting around with just the engine running for ages does not seem to work for me. Once the engine is turned off the sensors will go non-operational and the graph testing will be unavailable even if you start the engine again.

I do have the detailed information of exactly how a NOx sensor (what I have called the probe) works and what a NOx AdBlue system does, but it's essentially generic, and without knowing exactly what Mercedes has got up to with their software, hardware and firmware updates it's hard to know what the incompatibilities are if you are trying to save money and not just blindly replace with new parts. As mentioned before, there are many posts on NOx sensor problems, but most seem like a lot of guesswork or just general statements with little really definitive statements explanations of what was really wrong. There is little doubt that NOx sensors will fail over time (normally the heating element), but they can also be damaged by condensation pooling in the exhaust contacting them (normally the rear one) on some vehicles or just impaired by soot (DPF faulty[?]). Just blindly replacing the complete control unit and the sensor (probe) every time seems very wasteful to me.

So, I know the two sensors communicate with the N3/9 CDI Control Unit and the N118/5 AdBlue Control Unit over CAN I (CAN High and CAN Low). I can check the wiring via the plugs and X30/38 Potential Distributor Electrical Connector (CAN I), and attach an oscilloscope to monitor the CAN signals etc. etc. However, if that all looks reasonably OK (as it seems to) there has to be some incompatibility or fault somewhere that is not necessarily a bad NOx sensor(?). I should probably also add that I changed the O2 sensor on this vehicle about a year ago and used the Xentry teach-in processes for the components that needed it after the installation - all OK. I have also checked the DPF, and that seems OK as well.

Note also that I am aware that the AdBlue system can be mapped out and that there are NOx emulators and control board replacements that can effectively erradicate this flawed system. However, I believe they are all illegal.

Finally, you can probably tell by the lengths of these posts, I am retired now and have lots of time, but little money. It's far more beneficial to me to spend a lot of hours trying to bottom-out this fault, than to spend a lot of money with a garage.

......I said 'finally', but note that Supernoodle just posted some good information, so here's the reply to that:

I have swapped the control unit and sensor combinations as combined units. The 05 control unit has an NGK 11A sensor attached, and the 03 unit has a third-party replacement attached. Consequently, it looks like the same fault is being generated for each control unit that is not generating any fault in the rear position.

I am expecting another 05 control unit with third-party sensor in about a week (a replacement for a faulty one - see the graphing of that one attached for info), so I will see what happens with that as well.

Your info is very useful, and I need time to think about it. It might well be that the car has not had time to 'learn' things properly and reset the 751 factor value, albeit that I have done all I can see in Xentry. I may try running it longer than a 20 mile run with these sensors before testing again. It's possible that I may try resetting control units and 'teaching in' a few more parts of the system as well . I have programming facilities and CAN monitoring, so I can get into that later.

Thanks to all for your assistance so far.

Best Regards:

Edward

 

[ajlsl600]

Reading with interest. Hope an a to z on nox and sensor, module diagnosis/remedy comes out of this..
Good luck

 

[EdPhil]

Thanks. My hope was that somebody here could quickly direct me towards that a to z. It's doubtful that I can dig out all the details to expert level, but I do have the facilities to dig deeper and already have a lot of other useful information for this vehicle. I will keep this thread updated as I find out more.

One thing that does cause a few problems is that the Mercedes documentation (especially part numbers) does not always accurately reflect both my cars. That's not unique to Mercedes, though, as I have experienced it with Fords, Vauxhalls, Rovers, Jaguars, Audis and Volvos in the past.

Best Regards:

Edward

 

[ajlsl600]

There will be things upgraded and modified during and in production that even dealers may not be properly aware of "the well I never saw that before" syndrome.

 

[Steve@Avantgarde]

Lots of information here, some of it useful.

What are the fault codes? Lets start there.

 

[EdPhil]

Thanks for replies chaps. I am working on other garden projects whilst the sun is shining and don't live with my phones or computers with me or on all the time, so I am unlikely to pick posts up immediately. I expect to get hands-on with the vehicle again next week.

Steve: I think the basic question I am asking is getting lost a bit in all my dialogue, so I will try to precis that detail from above:

The annoying fault is 'P2201E3 The NOx sensor 1 (cylinder bank 1) has a malfunction', and this keeps occuring for various sensor and control unit combinations that look to me to be OK from testing and do NOT generate error codes when fitted in the downstream position.

Digging a bit deeper than fault P2201E3, the Actual Value 751 Factor for adaption of component 'NOx sensor upstream of SCR catalytic converter' is showing a value of 'Fault: Error details: Request out of range'. This may or may not be the true cause of the P2201E3 fault being generated.

What I really need to know is exactly where the value 751 Factor is being derived from and how to correct/reset it.

I don't know if it's a preset value or a learned value from other parts of the exhaust system or whether it may be caused by hardware and software/firmware incompatibilities between different versions. As ajlsl600 has intimated, an a to z on Mercedes nox and sensor, module diagnosis/remedy would be an idylic source of reference should somebody have one or be able to direct me to such a source.

It's evident from the detail in the images above that the value 980 Factor for the downstream sensor is set at 1.00 and that value (possibly) is applied as the correction value to the downstream sensor (whichever I fit in that position, so it does not look like it's derived from the sensor). The upstream sensor has a correction value of 0.00 applied to it, but I don't know if that's just by default.

I can dig a lot deeper myself, but it will be a lot quicker and take less time if somebody already knows what the value 751 factor is all about. Educated generalisations and guesswork can help, but it's still not definitive.

Best Regards:

Edward

 

[Steve@Avantgarde]

As I would understand it, that is embedded in the software within the ECU, a triggering margin for limp mode would be my assumption.

Ill level with you, its not something that I would, and I doubt any other indie or dealer would be too inclined to investigate, simply because, its a reference value for component failure and we're only interested in the failed component itself, its performance and the test path to confirm its failure.

That is done via the Tests button on Xentry (Ill assume you are using a clone machine with no pass through to Germany). The test button is very helpful as, as long as you do the test as instructed and fill in the correct answers to the tests questions, it will lead you to the front door.

Of course with all Nox issues and any replacement sensors/components you have to update the software of the engine, gearbox, SCR and powertrain, carry out a full adaption, manually regenerate the DPF and then you'll know if you have actually fixed it.

Long and short, I've been around since day dot on these SCR systems, and never has repairing the sensors and other technical fudges ever fixed them in the long term, only replacing the sensors and running the latest version software.

 

[EdPhil]

Steve:

Thanks very much for your insights. Much along the lines of what I have come to surmise - the latest software and sensors (or is it really just the control units[?]) should work - at a cost.

I just have a bit more time than money, though, and it's interesting to try and really understand what's going on. All the different versions of the sensor control units and software versions must work or have worked together in some combinations (albeit unsatisfactorily) at some time. Mercedes original 'frigging' of the SCR system to work differently at test time and the consequent mods when they got found out will have caused various incompatibilities and knock-on affects to other components and engine running characteristics. It's very unsatisfactory when there are lots of people out there selling used spares, reconditioned old control units and sensors or new sensors to wire in, when they will not work properly and may introduce further problems because there is little documentation or understanding of exactly what these incompatibilities are, especially with the NOx control unit versions.

You are correct about the Xentry I have being a clone (at Dec 2021). Also, the last time my vehicle was at Mercedes was in late 2020 when, I believe, they applied relevant updates including the diesel software update, which I have confirmed by checking on-line. Mercedes did not change any NOx sensors or other components at that time. I was therefore (maybe naively) originally (some time before this post) just hoping to find information on what version/s of NOx control units should be compatible with the diesel software update, as the originals fitted were Q 02 5WK9 6681C units and these kept generating spurious faults.

So, my current intention is to fit two Q 05 5WK9 6681D control units and then update, adapt and test - I hope to start this next week.

Would you happen to know which are the very latest versions versions of the sensor control units (printed on their backs) and the software updates for the other relevant control units? A print of a Xentry quick test log of a CLS 218 350 that has had all the latest updates applied should show details for all control units fitted(?), and would be ideal for me to compare with the current configuration of mine, which I am happy to share with you.

Best Regards:

Edward

 

[Steve@Avantgarde]

Hi Edward,

I always go by the MB part numbers rather than the version variant numbers and I would advise using that as your guide from this point.

For example, when you have a NOx fault code you hit the test path button usually the first thing it says is to check the software versions and update if required or it will simply say to check the basic electronic function eg, live earth and signal and then give you the part numbers you require to repair the car with its new software.

If your car hasn't had any software since 2020 then there will be some available as their have been multiple updates since then and that may be part of your tail chasing problem it sees a mismatch between component and software. That's why you cant replace just the one sensor, they have to be done in pairs always.

I would be able to share the latest part numbers with you.

Steve

 

[EdPhil]

Steve:

I take your point, but that's really what all my 'waffle' above is about, the same MB part number seems to be applied to various models of the Continental control units and although they all look the same and have the same MB part number written on them, they are obviously different because of the additional details applied. MB dealer new stock may have the latest versions, but who knows what that is? - Mercedes don't seem to want to tell me! Some people (not necessarily MB dealers) seem to have old stock or are combining different, possibly incompatible versions either as new or reconditioned units.

When I first started having this problem I was informed that it was only necessary to replace the actual sensor, not the control unit as well - pretty logical to me. As I got more into it, I realised that most people had no really detailed information or understanding about this: it's not that simple, and the diesel software update is the real source of many NOx sensor problems. It will also have changed a lot of parameters in the engine management and exhaust monitoring systems. My original control units are old versions and there are probably incompatibilities with them and whatever the diesel software update did to the other systems

As the actual sensor itself is not much more than a device that generates a varying electrical signal based on NOx levels that is sent to the control unit, there should be no need for SCN coding if only a new sensor is wired-in.......Why they did not separate the two components and just connect them with an eight-pin plug seems an engineering oversight, as I doubt each sensor is adapted individually to its control unit during manufacture(?).

What initiated this post in the first place was that I am still getting the same error code that I did with the old control units as I am with newer ones, Q05 being the latest I have, but I am using that in combination with a Q03. The Actual Value 751 factor error seeming to be a common factor as well, but expecting a quick answer on where that is derived from may have been too optimistic.

I am sure you are correct that replacing both sensors (complete sensor and control unit modules) with new MB units and updating all the software is probably the quickest and surest way to cure this - I am concerned about the cost, though. I just need to convince myself that it's really necessary, as I may, when both control units are Q05, already have a working combination with the 2020 software, as long as all necessary adaptations, resets and SCN coding for the new control units is performed. I may not be able to perform the SCN coding offline without a lot of investigation and manual codework, so if I hit a point where I get stuck, I may book a visit to your workshop, as you seem to be 'on the ball' and aren't that far away. Anyway, I hope to get the car in my garage later this week to get Q05s installed as a pair and then I will really get into it.

MB part number for the sensors (complete sensor and control unit modules) is A 000 905 34 03, and that was still valid in my offline Xentry at December 2021 (data status). I have seen nothing that indicates this part number has changed(?).

I can supply you with a print of a quick test log detailing all the software part numbers and versions applied to all the control units, but it's probably not worth it(?). However, in short: the latest version on N118/5 is 19/13 00; Y3/8n4 is 18/40 02; and, N 3/9 is 19/27 00. It's not clear to me yet that any other units would be directly affected.

I must admit, that I don't really like updates that rarely seem to make things more beneficial for the driver, and tend to avoid them where possible, unless I know exactly what they are for. I got caught off guard in 2020, when I was only expecting trailer module updating to be applied. My 2011 E class has not been subject to updates since 2014 and it very rarely has a problem, runs perfectly fine, and is still a pleasure to drive.

Thanks very much for your input. Much more constructive than 'Shouldn't have bought one of them in the first place' advice I have seen some people give others.

Oh, I just remembered: I did consider that old AdBlue fluid may have been the problem and so I drained the tank and part filled with fresh. It's only supposed to have a shelf life of about a year, so if you don't do a lot of mileage (like me nowadays) it can get way past that date. When purchasing a fresh supply, I found that various places were selling old stock with only a few months left before expiry - something to watch out for.

Best Regards:

Edward.