Several months ago a BMW owner, who has a car with an N52B30 engine, contacted me. The performance of the engine was unsatisfactory – uneven. Irregular performance was observed – unstable idle, and also persistent vibration.


Error messages, recorded in the error message memory:

  • 2C6C Lambda probe behind catalytic converter 2, system check, and
  • error messages regarding the performance of CO catalytic converters, bank 2.


Performing diagnostics, using ISTA, the owner found interesting nuances:

As we see:

  • the offset type LTFT of bank 2 is 0;
  • voltage of the regulating probe is above 2.00 V, it means – Lambda >1.00;
  • the voltage of the control probe is close to 0.

Both Lambda probes (wideband and control probe) confirm lean fuel mixture, but the engine management system (DME) don’t try to fix the situation in any way.


Both Lambda probes of bank 2 recently were replaced with new ones but didn’t change the situation.

Specialists of local service recommended to replace CO catalytic converters or ignore the problem – say, this defect cannot be solved. Electronics “specialist” already programmed out activation of EML indication due to error messages, mentioned before and claimed, that now everything is in order!


My comment regarding such diagnostics (suggestion) – it’s faulty, because:

  • decreased performance of CO catalytic converters (even if it’s 0) can not be the cause of decreased voltage of the control probe for a long time;
  • decreased performance of CO catalytic converters (even if it’s 0) can not be the cause of error messages regarding wideband probe and turned-off offset type LTFT.

Turned-off LTFT and evening of the cylinders efficiency – 100% confirmation, that the engine works in limp mode!


Performing the diagnostics remotely (helping the owner of the car, who is quite skilled with ISTA itself), I came to conclusion:

  • deleting of old adaptations and creating new ones don’t improve the situation;
  • test of Lambda probes ../F9/F3 indicates the normal performance of all probes.


Turned-off offset type LTFT means, that by some reason the engine management system has not succeeded to reach correct indications of probes in idle. There can be different causes for such a problem:

  • problems with fuel pressure (too low/high);
  • problems with an air supply (incorrect data of air mass meter, air suction in the inlet manifold, EGR valve etc.).


Taking into account, that offset type LTFT of other bank is correct, elements, which are common for both banks, were excluded from the “guilty” list:

  • anything, what relates to a fuel supply (fuel pump, pressure regulator);
  • air mass meter, air suction in inlet manifold, EGV valve etc.


Rough run data don’t show huge differences of mechanical efficiency in any of cylinders of a 2nd bank, also injectors were taken out from the list of suspicious elements.


Cause of the problem was identified when performing a visual inspection of the exhaust system. It was found, that the connection of the pipe of the exhaust system is not air-tight (marked in the picture).

Most interesting – this connection is located BEHIND control probes, but we have to keep in mind, that the speed of the exhaust gasses is very low and pulsating (RPM of the engine – low). As a result of these reasons – the air was sucked into the exhaust system and “mowed up” the pipe and causes problems, described before.

Position No.5: control probes of the 2nd bank, damaged connection marked with red.


Due to oxygen in the system:

  • voltage of the control probe was decreased, DME tried to fix the situation, making the mixture in 2nd bank richer and recalibrating wideband Lambda probe;
  • when the range of regulation were exhausted and max threshold was reached, the error messages regarding wideband probe (because it was not trimmed successfully) were recorded; offset type LTFT of the corresponding bank were turned off; evening of the mechanical efficiency of cylinders was turned off; trim of the wideband probe of 2nd bank was turned off.


In the same time – when the mixture was rapidly enriched by 10% (test of Lambda probes), the mixture became rich also in the range of control probe area, and the control probe indicated correct voltage for a short time – defect as if disappearing.


That’s how the sealing rings of the connection of the exhaust system looked after dismantling:

Exhaust system after repair:

After repair, the old adaptations were deleted, the engine was re-adapted. DME set up the correct offset type LTFT (different from 0), the voltage of the control probe in idle is around 0.70 V; vibration and uneven performance of the engine disappeared. Error message list of DME – empty.

One more interesting nuance – if the owner of the vehicle would agree to replace the CO catalytic converters, most probably, the real cause of the problem also would be solved. Most probably, the “specialists” with a clear conscience would claim, that they replaced to CO catalytic converters, and the problem disappeared! ”We said that!” Just one small nuance – due to faulty diagnose the owner would unnecessarily spend EUR 500 .. 1000…