Wide-band probes. Hidden defects. Part 2.

The defect, mentioned in this entry, is quite rare. But in my practice are several cases with exactly this defect. It is very hard to identify this current defect – in all cases, all possible “cures” were tried – replacing ignition coils, spark plugs, injectors, fuel pump(s). And all without results.



a) uneven performance of the engine, unstable idle;

b) unbalanced cylinder efficiency in idle, vibration;

c) error messages regarding (usually one bank) cylinder (one or several of this bank) misfire.


As already mentioned before, replacing ignition coils and injectors don’t help.

At the same time – both banks are able to maintain the fuel mixture, integrators are performing normally, offset and multiplicative adaptations – correct.

N42; N46; N52; N54 engines, also N43/N53 engines, if they ARE NOT performing in Stratified mode (for example, error messages regarding NOx system are recorded) won’t have any error messages regarding Lambda probes. If the N43/N53 engine performs in Stratified charge – the error messages regarding damaged Lambda probe (plausibility) will be recorded, but, when the engine will switch to Homogeneous mixture, “damaged” Lambda probe will report normal indications (Nernst cell internal resistance, and integrator will perform as if normally – the engine will be able to maintain Homogeneous mixture).


Where the problem lies, how to discover and eliminate it?

In N series engines, Bosch wideband probes LSU 4.2/4.9 are used before CO catalytic converters. Each probe has been tested and calibrated by the manufacturer. In each plug appropriate (to current probes parameters) resistor is soldered – it defines the sensitivity of the probe if the Lambda of the fuel mixture is different from 1.00.

In the picture: connection of LSU 4.2. Calibrating resistor (30 .. 300 Ohm) connected between red and green wire.


Trim resistor inside the connector. LSU 4.9

In the picture: LSU 4.2 outgoing current vs Lambda

a) blue – factory-defined curve, if the calibration resistor is connected correctly;

b) red – curve, if the connection of calibration resistor is interrupted (open circuit. Example of Rcalibration = 68 .. 82 Ohm).


Example: Lambda probe connection for BMW N series engines (N53). Calibration resistor locked between Pin5 and Pin6 (X62101).


In the situation, when by some reason (for example, humidity in the plug, resistor connection damaged because of vibration) the calibration resistor circuit is broken, Lambda probe becomes much more sensitive to changes of Lambda. It is typical than in case of Lambda = 1.0 the impact of this calibration resistor is 0, which means, that in case of the Homogeneous mixture the probe will show the correct value.


Why does the engine not work properly if with Lambda =1.00 the probe shows correct value?

Problem No.1: the engine management unit tries to maintain the correct mixture in each of banks. For example, if slightly enriched mixture is detected, MSV/MSD (and any other engine management unit) will try to correct the situation, decreasing the fuel supply; and vice versa – if the measured fuel mixture will be lean, the engine management unit will enrich the fuel mixture (closed loop system).

For example, if the measured Lambda is 0.99, the engine management unit understands: the amount of fuel has to be decreased by 1%, to reach the perfect fuel mixture. But, if the calibration resistor connection of the probe is damaged, it will identify much larger difference from ideal Lambda value (1.00) as is true; for example 0.98 (double difference, comparing to true one). When the engine management unit will detect a much higher difference from the ideal one, it will make the decision to apply much higher compensation than actually needed. As a result – in the typical situation, fluctuating process, during which the engine management system tries to stabilize Lambda, takes place. Even, if it succeeds, the RPM in idle is fluctuating for a longer while, Lambda is unstable (fluctuates around 1.00), misfires are possible.


Problem No.2: both in idle and on the run cylinder efficiency measurements are performed, and the fuel amount, injected in each of cylinders, is corrected, to reach max even performance of the engine. Results of these tests are relative (it means, the cylinder efficiency is determined in the relation of one cylinder to other). Accordingly, if, for example, chemical or mechanical efficiency of cylinder X is for 5% less than for cylinder Y, the unit management system makes decision: increase the fuel amount, injected in the cylinder X, for example, for 3.5% (two thirds of the difference – to avoid overcompensation), repeat the test to reduce further disbalance. Theoretically – the results of next test have to show 1.5% difference, then, applying correction of 1.0%: 0.5% difference from ideal, etc. This algorithm works good, but – not this time.

What happens, it the engine management system measures as if 5% difference (which actually is two times less, only 2.5%)? It tries to correct the situation, redistributing the amount of injected fuel in cylinders for 3.5%. If before there was a slight lack of fuel (-2.5%), then now, after increasing it for 3.5%, the amount of fuel will be too much for (+1.0), such overcompensation is a safe condition to initiate the fluctuation process. The problem of the opposite “polarity” will be confirmed by the probe, showing high positive efficiency (lying again – showing the too large difference from perfect value). The unit management system will try to save the situation but will make it even worse – overcompensation (the fluctuation process will start).

As a result – the system of individual efficiency measurements os cylinders is totally paralyzed, cylinder performance is characterized by huge disbalance, misfires.

The defect is even more hidden by the fact, that, for example, deleting the 2nd group of adaptation, the engine performs without problems (till Lambda probes are not being heated), Rough run menu shows, probably, permanent disbalance, but no misfires, shivering, fluctuation of idle observed. The defect is as if gone, but, when the engine starts to perform the adaptations, is stubborn returns.


The effect of the problem is inversely proportional to nominal of calibrating resistor – as larger the nominal (closer to 300 Ohm), as smaller the problem. But, regardless of nominal of calibrating resister, the normal performance of the engine is disturbed, but Stratified charge mode possibility – totally eliminated.


How to detect this defect?

1.Perform the test of Lambda probes: ../F9/F3. In case of this defect, the wide-band probe test will show not 0.90/1.10 (or close to these values), but strongly different values – it means, around 0.85 .. 0.80 (0.80 is a minimal value, which can be measured by MSV/MSD) and 1.15 .. 1.30 or even more;

2. Lambda for the N43/N53 engine, when it will try to perform in Stratified charge mode, will be highly increased (instead of 2.50 .. 3.00 reaching even max displayed value: 16);

3. disconnect wideband Lambda probe, measure resistance between corresponding contacts: 5 and 6 (N53), 2 and 6 (N42), etc. This resistance has to be within 30 .. 330 Ohm. If the resistance cannot be measured – the probe (as in points No.1 and 2) has to be replaced.


Related entries

MSV70/80, MSD80 and Lambda probes. Hidden defects

Management of Lambda probes

Lambda probes. Wideband