At the first moment, you can think – what does the error messages 2A26 and 2A27 to do with the NOx sensor.


2A26 catalytic converter, conversion during shift operation

2A27 catalytic converter 2, conversion during shift operation


Both error messages refer to CO catalytic converters: their insufficient efficiency in Stratified charge mode.


As we know, for the control of the efficiency of CO catalytic converters (N43/N53 series engines are not exceptions), the control probe(s) is (are) used.

In case of Stratified charge, the NOx sensor is used to control the efficiency of the CO catalytic converters – this is the common denominator.


In time, when the engine works in Stratified charge, the NOx catalytic converter is regularly (after 15 .. 55 seconds) regenerated (cleared from sulfur) – for a second (sequently) the portion on the fuel is dropped out in the exhaust (Lambda drops to 0.9 shortly, it means – in the exhaust around 10 % of unburned fuel is dropped). The task of the CO catalytic converters – burn part of this fuel (other part will be burned by NOx catalytic converter). If CO catalytic converters are in working order, they both reduce the number of exhaust gasses during these regeneration cycles and burn the fuel – increasing the temperature till around 600 oC is exactly what is needed for NOx catalytic converter to regenerate.


If CO catalytic converters work, then they burn fuel, using the oxygen for this cause. Accordingly – directly after finishing the regeneration cycle (working) CO catalytic converters will have reduced content of the oxygen: reduced Lambda. Measuring the length of this “drop” of Lambda, it’s possible to detect, how efficient the CO catalytic converters work.

As Lambda during these regeneration cycles changes in a wide range, the control probes can not be used for this cause, because they work in a narrow range of Lambda: 0.99 .. 1.01. But also the NOx sensor knows, how to measure Lambda (it contains tools to measure both narrow-band and wide-band Lambda)!


Measuring the performance of the CO catalytic converters is done by “fail/pass” principle. MSD measures time of decreased Lambda after regeneration cycles and counts fail/pass measuring cycles.

The error messages 2A26 and 2A27 can be recorded in the following cases:

a) damages of the CO catalytic converter(s);

b) long driving in a range of low load;

c) damages of the NOx sensor;

d) leaking injector(s).


With (a) situation it’s clear – CO catalytic converters have to be replaced (in this situation also the error messages 29F4 and 29F5 have to be recorded, the engine has no other problems), I will pay more attention to situations (b), (c) and (d).

(b) I have managed to “catch” the error messages 2A26 and 2A27, driving for a long time with a very low load to the engine. For example, driving evenly with 45 .. 50 km/h for more than 2 .. 3 hours. Yes, the temperature of the exhaust system is around 250 oC, but, obviously, the CO catalytic converters itself are too cooled down (not warmed up) to perform their task.

The last (c) situation is more interesting. If the NOx sensor is damaged – it can not correctly measure Lambda, but has passed self-test procedures (don’t report any error messages regarding self-test), the situation, that DME identifies a false error message of the CO catalytic converter(s) because the NOx sensor don’t inform MSD regarding correct (rising accordingly) Lambda after them, is possible.

Yes, most probably – during time MSD will detect incorrect Lambda indications of the NOx sensor. But it takes time to determine such an error. And, if the NOx sensor will sometimes pass this appropriate test, it can happen, that MSD will suppose the NOx sensor works appropriately, but the CO catalytic converters, instead – are damaged.

Also, check injectors data at ../F5/Shift+F6/F5. If correction for any of injectors show lowered values (-0.50 mg/stk) or difference between injectors correction values within one bank is higher than 1.0 mg/stk, the cause of corresponding error could be leaking injector.


What to do, if in the error message memory of the MSD the error messages 2A26 and/or 2A27 are recorded:

a) if there are also ANY error messages regarding NOx sensor – the sensor has to be replaced, the error messages have to be deleted, the engine has to be re-adapted and the situation has to be observed repeatedly;

b) if no error messages regarding the NOx sensor are present, also no error messages 29F4 and/or 29F5 – check the error message status. If they (2A26 and 2A27) are passive – delete them, check the performance of the engine in Stratified charge. Mandatory observe the NOx sensor. If its Lambda values don’t correspond to values of the wide-band probes (the difference is beyond 10 .. 15 %) – check the air suction in the exhaust and evaluate the conformity of indications of the probe or sensor. If sometimes NOx is on Offline mode (when it has to be Online), replace the sensor – it has a hidden defect (unfortunately – quite an often problem). If a strong difference of the cylinder’s efficiency or Lambda of the banks is noticed – prevent the problem (typically – leaking injector, damaged Lambda probe or damaged NOx sensor).

c) even if there is full bouquet with the error messages regarding CO catalytic converters (either both error messages regarding one catalytic converter or at least 3 from 4 error messages) – check if no other anomalies are present. Typically – incorrect mechanical efficiency of  cylinders in idle (../F5/F7); the error messages regarding any injector of significant corrections in idle (../F5/Shift+F6/F5, first 6 lines); significant correction in the range of the average load (../F3/Shift+F5/F3 and ../F5/Shift+F6/F5, bottom part of the menu); problems with maintaining the fuel mixture (Lambda in Homogeneous mode is not 1.00 .. 1.01), the error messages regarding trim of probes or functionality, etc. Any of error messages, mentioned before (cause – problems of injectors, probe or NOx catalytic converter) can cause the error messages regarding the performance of the CO catalytic converters. But the truth is the CO catalytic converters are either overloaded or DME receives incorrect data.