12C401. Plausibility: signal activity too low

Attention – in this entry you will find information regarding nuances, which have to be taken into account if NOXEM 402 is installed. If your car has an OEM NOx sensor, this error message typically means the damage of the cell of the NOx sensor (it’s clogged).

If 12C401 error message is recorded directly after installing the NOXEM (re-adapting of the engine: during first hundreds of km):

  • DME has noticed an unexpectedly good performance of the NOx catalytic converter, but “for security” has marked, that the data of the sensor are suspicious. Unlike the first generation of N53 series engines, the 2-nd generation (which is installed on F10/F11), the registration of replacement of the NOx catalytic converter is not intended. Accordingly – there are no options to inform the DME regarding the repair of the NOx system. In this situation:
    • delete the error messages;
    • delete the adaptations of the NOx sensor, as described here.
  • for some reason, the engine has “stuck” in Homogeneous mode. A more exact description of this situation will follow.


If 12C401 error message is recorded later (after several hundred, thousand or tens of thousand km), a typical reason for recording the error message – for some reason, the engine has turned off Stratified charge mode and for a long while (longer than necessary) is working in Homogeneous mode.

Note: in this situation, the 12C401 error message is not critical and doesn’t cause malfunctioning (Stratified charge). If the engine allows using of Stratified charge, the data sent by NOXEM correspond to ones, which DME “expects” from an exhaust, which is in perfect technical order.


Why is 12C401 recorded?

NOXEM informs DME regarding the perfect performance of the exhaust. DME accepts such indications till the moment, while it has no proof, that actually the performance of the exhaust is poor (or it has to be poor). For example, if DME mathematically calculates the contamination with the sulfur of the NOx catalytic converter (it has not been cleaned for a long time – no desulfation sessions performed), identifies the uneven performance of cylinders, insufficient performance of CO catalytic converters and other problems (more detailed info will follow), this information is contrary to ideal indications of NOXEM. DME draws conclusions that, obviously, something is trying to fool it (or the sensor has any damages).

In the current situation, this error message does not interfere with the engine to work in a Stratified charge, even more – you can use it to your advantage. How?

Unfortunately, the F series owners have very limited access regarding work conditions and “health” of the N53 series engines (much more limited than for E series N43/N53 series engine, detailed info here). ISTA has no information for the engine performance mode, contamination of the NOx catalytic converter or the status of the NOx sensor. Not even talking about learnbits, misfire counters or other more specific things. Accordingly – the owners of these cars don’t even know, if and how often the engine uses the fuel-saving Stratified charge mode. In this case, 12C401 is a good indicator, that you have to pay attention to the “wellbeing” of the engine.


What are the typical reasons, why the N53 “gets stuck” in Homogeneous mode?

1. Contamination of the NOx catalytic converter. This is the most typical reason, why MSD87 stops using a Stratified charge. The obstacle, that ISTA does not delete the contamination data of the catalytic converter (when deleting the engine adaptation data), is worth mentioning. It means – if ISTA is used to re-adapt the engine, the NOx catalytic converter has to be cleaned, performing the desulfation session! The desulfation session has to be performed also in case if the NOXEM is installed (true thou – not so often as with OEM NOx sensor installed, but still – it has to be done). Or – use INPA to delete the adaptations (and contamination of the NOx catalytic converter, instructions here) and must make sure, that the contamination level of NOx catalytic converter after deleting is indicated as 0mg. So – if you regularly don’t drive with at least 110 .. 120km/h, check the contamination of the NOx catalytic converter using INPA, or find the option to perform faster drives!

MSD87 (unlike MSD80/E series N53) has NO error message regarding contamination of the NOx catalytic converter, and there is no service procedure intended to activate the forces desulfation session. When reaching critical contamination of the NOx catalytic converter, DME turns off the Stratified charge without any notice to the driver.

2. Insufficient performance of the CO catalytic converters. If DME detects the insufficient performance of the CO catalytic converters in Stratified charge mode, this mode is turned off for some time. After a while, DME restores the Stratified charge and will measure the performance of CO catalytic converters repeatedly. The error message regarding the reduced efficiency of CO catalytic converters will be recorded after quite a long time or even will not be recorded at all if the performance will be just slightly below the norm. Accordingly, the situation can be formed – the engine rarely uses a Stratified charge, but DME records no error messages. One more nuance – if the error messages of such type (and also the current ones) are deleted, they are recorded repeatedly only after several hundred (not sooner) or even thousands of km. There is no reason to be optimistic if these error messages as if “disappear” directly after deleting. What to do in a situation, if there are suspicions regarding problems of CO catalytic converters? Of course, the problems of injectors and increased oil consumption should be solved. You have to drive for a long time (at least 1 .. 2 hours) with the increased load of the engine (with speed of at least 130 .. 140km/h), of with a lower speed, but aggressively. Increased load (temperature) can give a positive impact on the performance of the CO catalytic converters (and at the same time – will clean the NOx catalytic converter form the sulfur). How to check the condition of CO converters, read here.

Attention – do not use the additives to the fuel, which are intended to “clean” the catalytic converters. They can damage the injectors, fuel pumps, Lambda probes, NOx sensor.

Evaluate pass/fail counters of CO catalytic converters (detailed information here). If the up-to-date indications (for several hundreds of km) are lower than the average ones registered previously – obviously, the performance of the CO catalytic converters has reduced significantly.

3. Problems with a fuel mixture. Most unpleasant defects, which do not cause an immediate recording of the error messages in the DME error message memory:

  • air-tightness problems of the inlet manifold. The most typical culprit: CCV membrane or the connections of pipes of the collector. DME regularly will rewrite the offset type LTFT, readapt the fuel mixture of banks, remeasure the injectors. If the defect is not so impressive to completely destroy the fuel mixture, there will be no error messages in the error message memory, but the engine will work in Homogeneous mode for a long time. Detailed information here;
  • injector problems. If any of the injectors sometimes intensively leaks in the closed condition, injects an incorrect amount of the fuel or the fuel atomization is unstable – DME will switch to the Homogeneous mode and will try to solve this problem for a long time, remeasuring the injectors. If it will not succeed, the readoption process will be restarted again and again. Sporadic problems of the injectors can paralyze the normal performance of the engine for a long time. Unfortunately, only a small part of the corrections of the injectors is available in INPA. ISTA – zero information;
  • problems with fuel pressure, its stability. These engines have two (sequentially connected) fuel pumps. Unfortunately, problems with the stability of the fuel pressure are not rare. The cause – both problems of the electromechanical low-pressure pump and stuck of the management valve of the HPFP. 

4. The Stratified charge can be turned off also due to the uneven performance of the engine. Most typical reasons:

  • misfires of some cylinder due to ignition problems or an incorrect beam of the injector (additional information here) or all/several cylinders due to poor quality fuel (or incorrect spark plugs installed, more information here). How to check misfire counters read here;
  • false alarm of the DME due to uneven road. The users of engines of the previous generation are very familiar with the error message 3104, which was recorded in the DME error message memory even when driving on the pavement. DME of F series records the error message (regarding uneven run) much more “reluctantly”, which doesn’t stop it from turning off the Stratified charge at first “suspicion”.

If you drive the uneven road or you have felt the uneven performance of the engine (reason – poor quality fuel), don’t be surprised, that the engine has turned off the Stratified charge. Instead, if an uneven performance of the engine can be felt quite regularly, it is worth to check the misfire counter with an ELM327 adapter. Yes, unfortunately, users of INPA and ISTA have no access to these misfire data.

5. Any other problem of the engine systems. Even as if not connected problems – the error messages regarding oil pressure, the functionality of the thermostat, cooling blinds, fuel pumps, Lambda probes, etc., immediately will “throw” the engine in the emergency mode. Remember – these DME have no insignificant systems! Any error message can cause unexpected consequences. If you see any error message in the error message memory – solve the cause of the problem.


And finally – if 12C401 is recorded for one or several times a long time ago, and the engine has no other error messages: there is no reason to worry. In turn, if the DME records this error message regularly and recently: find out the reasons, why DME turns off the Stratified charge mode.