MSD87 (for N53 series engines) regularly compares readings of all Lambda probes (both wideband; both control probes and the NOx sensor). In situations when it clearly knows (or DME is “convinced” that it knows), what should be the fuel mixture (lean, rich, or Stoichiometric with Lambda 1.00), DME performs tests/inspection procedures, during which it makes sure, are the readings of all Lambda probes (including the NOx sensor) identical.
The NOx sensor measures and reports to DME both data of narrow-band Lambda (which by the content corresponds to the readings of control probes) and wideband Lambda (which by the content corresponds to the readings of wideband probes).
12C801 correlation error means that in some certain conditions (more exactly, it can be found in the error message freeze-frame), DME has detected that the Lambda, reported by the NOx sensor, is different from the fuel mixture data, reported by other Lambda probes.
Situations in which DME can record this error message are different. Most typical cases:
a. when the engine runs on Homogeneous mode, the NOx sensor reports lean or rich fuel mixture;
b. when the engine runs in Stratified charge, the NOx sensor reports too lean or too (relatively) rich fuel mixture;
c. when the engine runs in Overrun mode, the NOx sensor reports too (relatively) rich fuel mixture.
The reasons for recording the 12C801 can be different – all kinds of problems with creating the fuel mixture, leaking injectors, mechanical problems, and the damage of the NOx sensor (it’s reporting incorrect Lambda probes).
The main thing to remember – it is essential to eliminate the causes of the problem, not to fight the consequences! The 12C801 is exactly that situation. Not eliminating the true causes of the problem, you will have to replace all possible components “in around.”
Most typical causes, why the 12C801 error message is recorded:
a. incorrect (actually) fuel mixture. DME considers that it has created the correct fuel mixture, but the NOx sensor sends data of as if incorrect fuel mixture (actually – according to the situation);
b. the fuel mixture is different in locations of Lambda probes and the NOx sensor;
c. the fuel mixture is correct, but the NOx sensor sends incorrect data of the fuel mixture.
Let’s see each part more in-depth – the most typical causes of the problems mentioned before.
Incorrect (actually) fuel mixture. DME considers that it has created the correct fuel mixture, but the NOx sensor sends data as if the incorrect fuel mixture
a. leaking injector (injectors). A leaking injector can cause both additional unburned fuel in the exhaust and incorrect trim of the wideband probes. A leaking injector poisons the Lambda probes with a large amount of excess fuel. Poisoned Lambda probes give incorrect fuel mixture data; as a result – DME can consider that the fuel mixture is correct (as “claimed” by the wideband probes), but actually, the fuel mixture is either rich or lean;
b. misfires. Due to misfires, a large amount of unburned fuel gets in the exhaust, it poisons (or even floods) the Lambda probes;
c. damages of the Lambda probes (both wideband and/or narrow-band), including – their poisoning or flooding with fuel and/or water (the steam condensation problem due to short driving distances);
d. mechanical problems – the exhaust is not airtight. If the air gets in the exhaust (especially pronounced – in idle), the fuel mixture will be incorrect – it will be different in each place of the probe measuring;
e. significantly different distribution of the fuel mixture between cylinders. In addition to the typical leaking problems of the probes could also be: partial clogging of the injector (reduced flow rate), also bad atomization (the fuel is not injected as a fine mist, but is flooded), also other defects of the injectors;
f. problems of the Lambda probes (and typically – also the injectors) due to usage of different fuel additives. Sometimes, a low-quality fuel is to blame – unfortunately, such problem can also happen for the well-known fuel station brands. Getting the foreign parts and micro parts in the injectors (and causing problems for them) is contributed by the fact that the fuel filter is not replaceable – it is intended for all life of the car.
The fuel mixture is different in locations of Lambda probes and the NOx sensor.
a. the air gets in the exhaust. The air in the exhaust can get via damaged connections of the exhaust system or the ruptures of it;
b. significantly different fuel mixture between cylinders. If the fuel mixture is lean in the part of cylinders, but in part – rich, the leftover of the fuel mixture burns in the exhaust. The control probes (narrow-band probes) do “sense” only oxygen (but they can not “sense” the fuel leftover); the wideband probes instead do “sense” both the air and the fuel leftover. If both – the air and unburned fuel – get in the exhaust, wideband probes are not trimmed correctly. Instead, the NOx sensor gives different (from wideband probes) data of the fuel mixture because it can “sense” also unburned fuel; additionally, the fuel’s burning continues also in the NOx catalytic converters (and it differs before and after converter).
The fuel mixture is correct, but the NOx sensor sends incorrect data of the fuel mixture.
This, most probably, is the simplest case – a damaged NOx sensor. But the most important – the sensor damage is already a consequence, not a cause. There always is a cause of the damage of the sensor probe! So the most primary task is to identify and eliminate the causes of the damage. In the opposite case – continuous replacements of the sensor (and also Lambda probes) are guaranteed. You can read here and here for more details regarding damages of the NOx sensor here and here. Shortlist of most typical causes of the damages:
a. the fuel in the exhaust – leaking injectors; misfires; injectors with very bad atomization; problems with the HPFP, Lambda probes; air-tightness problems of the exhaust;
b. water in the exhaust – very short drives (incorrect driving profile);
c. the oil in the exhaust – increased oil consumption (damaged CCV system, wear of the engine);
d. mechanical damage of the sensor – due to fuel explosions (because of previously mentioned problems) or the car drives on a barrier (gets hit on the exhaust);
e. usage of the fuel additives (to “clean” injectors, fuel system, “increase” the performance of the engine);
f. thermal shock, typically – driving in the deep/large puddle.
If the sensor is chemically poisoned (with fuel leftovers), it may “get well” if the main problem will be solved. As sooner it will be solved than the larger is the chance that the NOx sensor will be saved. So follow along with your car’s engine’s performance – if you feel the vibration, shivering, the smell of the fuel from the exhaust pipe, or other suspicious symptoms, do not postpone the diagnostics!
Very often, the problem is progressing in several steps. Most typical cause – problems of the injectors. Most often – some injector starts to leak. The problems usually get more pronounced during a cold start. The leaking injector both poisons the Lambda probes (in such a way making the situation even worse) and provokes misfires and explosions of the fuel in the exhaust. Unburned fuel poisons the NOx sensor.