Sometimes I receive e-mails with the following content:
“I installed a used NOx sensor, encoded a new NOx catalytic converter, there are no error messages in DME error message memory – everything is perfect! I didn’t have to waste a lot of money to repair the NOx system!”
Unfortunately till now – in all cases, after some time (quite often – very soon, sometimes – when the owner of the car can afford it) the radical solutions to repair the NOx system were required. In this entry – possible scenarios of the “cheap” solutions.

So: the solution – used OEM NOx sensor and encoding of NOx catalytic converter, to avoid the error message relating reduced efficiency of NOx converter: 30E9 for E series/180401 for F series.

First of all, a notice regarding the error message 30E9/180401. DME records this error message if the measured efficiency of the NOx sensor was very low for a long time. BMW understands, that there could be a low-quality fuel (for example, with high contamination of the sulfur) and other obstacles, which reduce the performance of the NOx catalytic converter for some time, so the short-term drop of the performance of the NOx catalytic converter will not record this error message. Even if the reduced performance of the NOx catalytic converter is identified, DME will ASAP try to desulphate (clean) the catalytic converter, Stratified charge is turned off for some time to wait, till the potentially low-quality fuel is used out. If necessary, DME allows the Stratified charge only for modes, in which the content of the NOx in the exhaust is not high, at the same time DME increases the temperature of the exhaust, trying to improve the performance of the NOx catalytic converter. DME does everything to “push out” the max of the NOx catalytic converter. Unfortunately, if the error message 30E9/180401 is recorded – it has had a serious reason. I do not know (in my practice there was no any case) no case when after encoding of the “new” catalytic converter, the error message 30E9/180401 would disappear for a longer time if the NOx catalytic converter is not replaced for real (of course – leaving the OEM NOx sensor).

Second, a notice regarding OEM NOx sensors. These sensors serve for several tens of thousands of kilometers. Sometimes even less. The sensors are very sensitive, they can be easily damaged by the moisture in the exhaust, mechanical impacts, also problems made by leaking injector (explosion of the fuel in the exhaust). And, if thinking logically – why would somebody sell a working NOx sensor? What are the chances, that the sensor is in working order if the vehicle is put to scrap metal or is destroyed in the car crash? Or anybody naively thinks that the owner of the car replaces the sensors (buys a new one, which costs EUR 400+) without any reason? It is not clear to me, from where a used, but well-working NOx sensors can appear!

Third argument. If you came to the thought of repair of the NOx system, most probably – all other techniques already have been tried. Typically – the ignition coils, spark plugs, and injectors have been replaced. The problems with an uneven performance of the engine are persisting for some time already. Accordingly – the NOx catalytic converter (due to its contamination or the error messages regarding the NOx sensor) is not been served – it’s not regenerated and desulphated, it is exposed to extreme overload (the fuel mixture in cylinders is incorrect, misfires – the fuel explodes in the exhaust, the exhaust has an incorrect working temperature, etc.). It is clear – even if at the beginning of the problem the NOx catalytic converter was in working order, now there is no illusion regarding its performance (at least, for me)

But, let’s suppose, that you chose to risk. If you get lucky?
I will keep my fingers crossed for this scenario. But – below find the list of the technical manipulations, which have to be done to evaluate the situation.

1. check, if the sensor works: read here

2. check if the engine uses Stratified charge: read here

3. even if both conditions are fulfilled – there is no ground for great optimism because of the error messages regarding the NOx system (specifically – both for specific defects of the NOx sensor and the error message 30E9/180401) are not recorded immediately! Instead – if at least one of the first two conditions is not fulfilled: there is no ground to be optimistic. The engine will continue to work in the emergency mode, “the project” has failed.

a) it is suggested to perform the performance of the used NOx sensor for several times, because of the OEM sensors have a typically hidden defect (and no error messages regarding it are not recorded), which is sporadic and sometimes can appear and disappear;
b) also the performance of the engine in Stratified charge has to be checked for several times – if the sensor has hidden defects, the Stratified charge may be turned on only sometimes – the NOx system, which is working in such mode, is equivalent to completely non-working;
c) if the engine uses Stratified charge, DME is measuring the performance of the NOx catalytic converter in this mode. It takes time to evaluate the performance of the NOx catalytic converter. Al least several hundreds of km, driving on the highway, several driving sessions. If you will drive in the city, the identification of an (insufficient) performance (and, accordingly, the recording of the error message 30E9/180401 of the NOx catalytic converter will be delayed. From one side – everything is as if in order, no error messages, from the other – DME is “getting ready” to record the error message 30E9/180401.