Unpleasant case

This entry is dedicated to all skeptics – is the NOx system truly critical for N43/N53 series engines to pass the TI.

Patient: E90, N53 series engine.


Here, the protocol from TI: by increased RPM, CO is 0.5 % (allowed norm: 0.2 %).

TI – didn’t pass.

Interesting thing: Lambda – correct; HC – low, which means there were no misfires; CO in idle – very low, so: the injectors are not leaking, the fuel mixture is truly correct. The TI inspector correctly had kept RPM increased for a long while to warm up the CO catalytic converters – without success.

Long lasting increased CO – so no unexpected misfire is one to blame.


The current engine – maintained in good order. Only error messages are regarding NOx sensor. Accordingly – Stratified charge is turned off.

The engine as if even works quite satisfactory (if we don’t count increased fuel consumption). What to blame in such a situation? Replace everything one by one?


Here, typical picture in history list: MSD haven’t been able to maintain a fuel mixture, the error messages regarding Lambda probe trim:

The fuel mixture menu displays as if perfect picture:

a) offset type LTFT: very close to 0 (so – injectors, air sucking – no problems);

b) multiplicative LTFT – small, so – pressures, air measuring, throttle – in order;

c) integrators – within the normal range; so – no sporadic problems;

d) reported values of wideband probes – correct.


Rough run data – correct. Of course, VANOS was checked – positions are correct, jittering – small; LPFP and HPFP – pressures are stable, the chemical activity of probes – correct. All other systems – in working order. I won’t even publicize these data – believe me, nothing interesting. Yes, Rough run sometimes shows small persisting unevenness, live data of injectors – the individual adaptations of injectors are turned off, but – that’s normal, taking in account there are the error messages regarding NOx system. The injectors are in perfect technical condition – no claims.


Only small note – the voltage of control probes is slightly lowered. Such a voltage can seen strange. And it also is strange (why – description follows below).


Increasing the RPM of the engine till 2000 (problematic region, in which the TI wasn’t passed):

we see, that in the 2-nd bank the fuel mixture becomes slightly richer,but in 1-st – voltage of control probe slightly drops (on becomes even more strange).

Here, in this menu, we see many adaptations haven’t been done (marked with red). The adaptations of a cold engine, Stratified charge and individual adaptations of the cylinders haven’t been finished.


When checking the NOx sensor, we have to conclude:

There are no wonders – it doesn’t work, don’t turn on Online mode. But, more interesting – the exhaust temperature (right side, bottom part) reaches untypically high values (picture taken, when 2000 RPM has been held for around a minute, the car was in an environment temperature, outside – at the yard, additionally – the engine haven’t warmed up completely).

What is the cause of such increased temperature? Incorrect fuel mixture (although the average Lambda of banks is exactly 1.00)! One of the cylinders “spits” an excess air in the exhaust, other – excess fuel. This air and fuel burns in CO catalytic converters, heating them up too much.


And here, a screenshot was taken by chance, when the engine runs idle:

Here, the voltage of control probes (in addition, it was like that for at least 10 seconds) is strongly decreased – incorrect. How so?

In true – it’s the average voltage of probes. The true signal looks like a rectangle. Cause of this problem – incorrect fuel mixture in cylinders: in one it’s enriched, in other – lean. Cause of this problem – adaptation creation, paralyzed due to problems of NOx system.

Why there are no error messages regarding CO catalytic converters? Hopefully, nobody has modified MSD software (and deleted the error message map) and the error messages are not recorded, because the DME hasn’t finished the basic adaptations.

In a normal situation, MSD finishes these basic adaptations during several minutes, but – if the NOx system is damaged – it won’t finish it ever.

Unfortunately – there are no quick and cheap solutions, how to solve this situation. The only way, how to restore MSD ability to create adaptations – restore the NOx system.

Note: MSD creates individual adaptations of cylinders (adjusts fuel mixture of each cylinder, evaluating its performance):

a) in idle (performing tests of mechanical efficiency);

b) while driving (performing tests of chemical efficiency).


Only after restoring the NOx system, MSD will resume creation of cylinder adaptations and will optimize the fuel mixture in each cylinder individually. Correct fuel mixture will decrease the load on CO catalytic converters, and accordingly – also the content of CO in the exhaust.