Rough run. Trim errors. Hidden cause

The most woe of N43/N53 series engines: uneven operation – is well known to all interested, enthusiasts and repairers.

Yes, sometimes spark plugs, ignition coils and injectors are ones to blame. But one tricky cause is hiding, and nobody mentions of it. What could it be?


But now – step by step about everything.


The chart describes the relation between NOx system problems and increased fuel consumption, rough run, shivering, fuel trim errors.

BMW N53 NOx problems chart

Factory made N43/N53 series engine uses following fuel injection modes:

  • Homogeneous (H);
  • Homogeneous lean (HL);
  • Stratified charge injection (Str).

Optimal fuel amount is calculated for each injection mode and current driving conditions using adaptations: LTFT (long time fuel trim) and corrections: STFT (short time fuel trim). MSD80 engine management system uses following methods for fuel management:

  • fuel amount calculation (using both: STFT and LTFT) for banks* for all working conditions (idle/low/medium/high loads) for each injection mode (H/HL/Str) separately;
  • additional fuel amount correction for each cylinder individually at Homogeneous injection or Stratified charge at idle based on cylinder’s mechanical efficiency tests (measuring speed deviations of the flywheel);
  • additional fuel amount correction for each cylinder individually at Homogeneous injection at low/mid/high loads based on cylinder’s performance tests (measuring Lambda deviations in the exhaust). These (injector’s actual efficiency/flow-rate) data are transferred to Stratified charge and Homogeneous lean injection modes and are used (when HL or Str are turned on) for the same driving conditions.

*cylinders 1/2/3 and 4/5/6 for N53 series engine; cylinders 1/4 and 2/3 (520i) or 1/2/3/4 (1/3 series) for N43 series engine.


If the engine has error messages regarding NOx catalytic converter and/or NOx sensor (you can find a list of possible error messages on the end of this article), MSD80/81 (engine control management system) turns off Stratified charge and Homogeneous lean fuel injection modes and as the result – fuel consumption increases. Modes mentioned (HL and Str) are actually the main reason, why the N43/N53 were developed at all. In alternative (actually emergency: Homogeneous injection) fuel injection mode many options are “cut down”, and the engine operates close as early M50 with Motronic 1.3 control system with a very narrow area of fuel correction range and situation is following:

  1. If the main (Stratified charge) fuel injection mode is disrupted, N43/N53 don’t perform the individual cylinders fuel corrections normally, fuel mixture becomes non-optimal and the engine starts to shake (even if average fuel mixture within the bank is perfect). In addition at low/mid/high loads N43/N53 also don’t correctly creates multiplicative adaptions (LTFT), MSD80 use STFT (short time fuel trim) only to maintain fuel/air balance.

Note: the factory made engine (with normally working NOx system) create offset type cylinder-individual adaptations at idle in Homogeneous and Stratified charge injection modes. Cylinder’s individual multiplicative type adaptations at low/mid/high loads are performed at Homogeneous mode and these data are used for Stratified charge and Homogeneous lean modes too (for the same driving conditions: RPM and requested load). Multiplicative type adaptations for banks are created in all injection modes: Stratified charge; Homogeneous lean and Homogeneous injection modes as well.  If the Stratified charge and Homogeneous lean injection modes don’t function, all logic structure and main algorithms of fuel management are destroyed, and the engine is forced to “pull out” these data, using only corrections (STFT/short time fuel trim).

  1. If by some reason the engine “jumps out” the range of corrections/STFT (usually just +/-10 .. 15%, depends on software release) longer than certain time (usually about 10 seconds), MSD80 engine management determines fuel trim problem and records corresponding error message (problems in first or second or even both banks, lean or rich mixture);
  2. Registering exact cylinder as the “culprit”, the error messages about fuel trim problems frequently are giving incorrect information to a person, who performs diagnostics. The cause of this error message is a simple mathematical overload of data register; the cylinder who suffers this, is the one, which injector has had more different injector flow correction coefficients (but not the cylinder who performs incorrectly);
  3. Because of not working individual cylinder mixture corrections (adaptations), the problem gets even more serious; it’s usually followed by single or even group misfire. If group misfire begins, the injector of “damaged” cylinder is turned off, closed-loop mixture correction using Lambda probes will be stopped for some time, and the situation gets even worse; EML indicator is turned on;
  4. At the moment, when control of Lambda (for a while) is stopped and all cylinders work with default (without STFT) fuel mixture, cylinder (cylinders), which has a more lean mixture, start to skip ignition cycles again and MSD80 registers additional error messages of a misfire.

In a while (or – switching on/of the power to MSD80) the closed-loop control on Lambda sensors is restored, but the engine continues the twitching. The reasons are not completely clear, because no one (except the team of developers) knows the exact control algorithm, but it look’s that the system stops to use recorded adaptations (if they are present). The system’s logic says: if these current adaptations bring to such incorrect mixture, they can not be used. Even more – at an idle run, the individual adaptations don’t resume anymore. As the result is a rough unbalance between cylinders for cold, warm, or even in all temperatures of the engine. This problem can be seen with INPA, choosing analog modules (press F5) and rough run sub-menu  (press F7). You can see very well, that individual adaptations don’t converge to 0, and cylinders have a permanent and large difference in their efficiency readings.

Deleting of error messages don’t improve the situation. You can delete also adaptations (including syncing of cylinders), the individual idle run adaptations are restored, but they continue only till next time, when scenario, mentioned above, repeats.


NOx system problems. Affected vehicles. 107 vehicles inspected. Data: MSD80 history memory log.

Disclaimer: data represents situation monitored in current area and driving conditions and may vary into other areas/driving conditions and may not represent data monitored by BMW AG. For illustrative purposes only.

BMW N53 NOx system problems graph


Properly working NOx system is a critical requirement. Restoring the Stratified charge (which fundamentally solves the problem) for N43/N53 series engines is not possible in  “cheap & easy” way. In addition:

  1. you can’t downgrade the engine to “older” version (version without Stratified charge) by reprogramming because for this engine the Stratified charge mode is a basic mode, which is not possible without functioning of NOx system;
  2. erasing of corresponding error message codes from error message map also does not helps. Error messages don’t appear anymore, but all problems remain;
  3. disconnecting the NOx sensor don’t help;
  4. the simple (by programming) simulation of NOx sensor is not possible (it’s connected with loCAN and does not receive necessary data from dCAN);
  5. if the insufficient efficiency of NOx catalytic converter is determined (30E9 error message appears), “cheating” with the registration of “new” NOx catalytic converter also doesn’t help – MSD80 realizes (very quickly), that the converter’s performance is inadequate and renews active status of 30E9.


NOXEM – solution for NOx system problems: NOx catalytic converter and NOx sensor.

Detailed information about NOXEM 129

Detailed information about NOXEM 130


P.S. Why the adaptations are critically important to correct fuel mixture? You can read more about this on adaptation and compensation topic, but shortly:

each component has it’s technological tolerance, usually not less than +/- 5 .. 10%. Accordingly, high-pressure fuel pump, injector, VANOS solenoids, sensors, etc. have such tolerances. Such inaccuracy (which are very common already to new spare parts, can overcome this tolerance, when parts wear out) in the worst case can sum in more than defined (allowed by STFT) 10 .. 12%. Exactly by this reason adaptations are used – this long-term additional correction (stored in engine management controller between driving sessions) significantly wider permissible tolerance of mechanical and electrical components.


And now the list of error messages for problem described above:

2AEC Nitrogen Oxide Sensor Self Diagnostics

2AF0; 2B05 nitric oxide sensor, heating

2AF2; 2B06; 2B09 nitric oxide sensor, Lambda linear

2AF4; 2B07; 2B0A NOx sensor, electrical

2AF6; 2AFB; 2B0B nitric oxide sensor, Lambda binary

2AF9 nox sensor, nox signal: coast mode check

2EAE NOx sensor message missing (timeout)

30D6 nitric oxide sensor, plausibility

30D8 NOx sensor, Sensor damaged

30DA NOx sensor, heating time

30DC nitric oxide senor, heating

30DE NOx sensor – Lambdaprobe before catalyst, Korrelation

30E0 NOx sensor, Offset

30E2 nitric oxide sensor, thrust test

30E4 nitric oxide sensor, aging

30E6 nitric oxide sensor, dynamics

30EA nitric oxide catalytic converter, sulphurized

30E9 nitric oxide catalytic converter, aging


Related entries:

Stratified charge performance check

Components of NOx system


How I perform adaptations

Replacing NOx sensor

NOx catalytic converter regeneration and desulfation