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Hi, New here. Normally used to helping others but now I need some help myself!! PLEASE.

S

snanceki

New Member
Joined
Nov 13, 2006
Messages
6
Car
R172 350, C215, Renault Zoe
I'm a tech myself but I'm looking for additional TSB or other input on a specific DTC issue, namely P2201 / 2214 on a R172 M276 (40 k miles). I understand the code, what it means, what may be needed to correct the fault etc. etc. but I suspect that there is a BIG back story on this. Cancel the code and all is OK for a while and then bingo back it comes. It appears that MB have been changing NOx sensor Pt No.s multiple times and that there is a "design" issue rather than a problem with the sensor. In short the NOx storage cat (NAC) needs to be regenerated signalled by the MIL / ECL illuminating. This engine is Stratified Charge Petrol with NAC rather than the current design full Ad Blue SCR cat. I've picked up that MB are keeping their cards tight to their chest and info on this is not being published via TSB, but by other more "secure" channels. Any info would be much appreciated should there be any helpful MB tech staff out there.
 
snanceki said:
..R172 M276 (40 k miles). I . This engine is Stratified Charge Petrol with NAC rather than the current design full Ad Blue SCR cat...
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AFAIK the M276 is a CGI/GDI petrol engine, while AdBlue is urea SCR exhaust-gas post-treatment additive for Diesel engines?
 
markjay said:
AFAIK the M276 is a CGI/GDI petrol engine, while AdBlue is urea SCR exhaust-gas post-treatment additive for Diesel engines?
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Hi Mark and thank you for your response.
Yes the M276 is a CGI (Charged Gasoline Injection) which is Mercedes (and others) speak for Stratified Charge (SC). SC is a new technology (at least relatively) that operates in a similar manner to Diesel except that it still has a spark plug and runs on gasoline. The main difference is that the combustion chamber is so designed that during the induction stroke a very weak mixture 50:1 (vs normal 14.7:1) is drawn in (air plus a first squirt from the DIRECT fuel injectors) and towards the end of this stroke the piezo injectors deliver a further squirt of fuel that is directed to accumulate adjacent to the spark plug, and is rich enough for the spark plug to ignite, thus setting off the complete mixture.
These engine run at relatively high compression ratio (typically 12:1) and are VERY much more efficient than a standard normally aspirated stoichiometric mixture engine. Higher compression can be used because the weak mixture can't detonate (or "knock") on the compression stroke, because the mixture is too weak to ignite.
However the reality is a little more complex and in particular these engines have a downside, namely that they put out more NOx (just like Diesel engines) due to the higher combustion temperatures. This extra NOx has to be controlled with a special storage catalyst (NAC or NOx Adsorbion Catalyst) aft of the usual 3 way catalyst (that deals with Hydrocarbons (HC) and Carbon Monoxide (CO)). This special cat on the R172 has to be regenerated once it gets full.
More recently, especially on Diesel engines, NOx is dealt with by a special catalytic converter known as SCR (Selective Catalytic Reduction) which uses Ammonia (produced from Ad Blue or Urea) injected directly into the exhaust.
The need to regenerate the NAC is signalled by the NOx sensor.
As I understand it the P2201 code means that the Engine Management ECU requires the NAC to be regenerated because excess NOx is being sensed, and signals this by illuminating the Check Engine Light (CEL). The regeneration SHOULD have been carried out automatically by the ECU temporarily enriching the fuel settings allowing additional O2 into the exhaust system.
Now this is where the mystery begins to unfold.
This vehicle has happened to fail an MOT for high lamda with a reading of 1.035 (usual range 0.97 to 1.03) but after a blast down the motorway passed with flying colours. Incidentally HC and CO were zero!) High Lamda as measured in the MOT means that the mixture is weak (or in MOT speak NOx is high) which of course is to be expected with a SC engine albeit that the NAC SHOULD have absorbed any excess NOx.
Interestingly the BMW N53 engine (which is SC) has a special allowance in the MOT test of 0.97 to 4.0!!) but the SLK quotes the regular 0.97 to 1.03) Could this be a mistake? I suggest so!
Reading the web the incidence of MOT fails due to high Lamda is significant on SC engined cars which is why I'm trying to see if anybody can add anything to the back story since there are reports everywhere of customers changing NOx sensors but not resolving the basic problem.
Furthermore I sometimes get a P2201 whilst other time P2214 (Bank 1 vs Bank 2) so IMHO the chances of two sensors going down is unlikely and I sense there is a design issue at the bottom of this.
Tomorrow the car goes in for this years MOT test. It will have a good run prior to the test. It will be interesting to see what transpires.
I'm not willing to payout in excess of £600 on a yearly basis to change the NOx sensors when I don't believe they are really at fault.
WSY?
 
The replacement part has a 2 year warranty so you wont need to replace annually...

I understand the replacements are a different, improved, design so are far less likely to fail.
 
greenmanslk said:
The replacement part has a 2 year warranty so you wont need to replace annually...

I understand the replacements are a different, improved, design so are far less likely to fail.
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Thanks for the reply,

So could you substantiate this please....replacements are a different, improved, design so are far less likely to fail!

Part numbers, MB or After Market. Manufacturer, Price etc.
 
Do you have access to EPC? It will show the original P/N and the replacement P/N (based on your car's VIN).
 
Hi
Quite often the updated part requires a software update to the ECU as well, this is certainly the case for the diesels.
 
This is indeed the case as I had to have mine coded in when I had a nox sensor replaced with a new ( and improved) part on my M276.
STAR might be needed I expect
 
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