The diagram shows why the compliance with lambda = 1 as strict as possible is so important. It could also be calculated how the carbon released from carbon monoxide (CO) and hydrocarbons (HC) and the oxygen from nitrogen oxides (NO_X harmless carbon dioxide (CO₂) can be formed and together with the remaining hydrogen (H) water (H₂O) and nitrogen (N) remains.

Basically, the control unit, together with the signal from the two-point probe, ensures exactly those parts that can later be reassembled into harmless substances when they pass through the three-way catalytic converter. If there is a deviation from lambda = 1, either too much CO or HC is present or NO_X has a maximum. Both are to blame for the fact that the respective pollutant(s) and the pollutant(s) to be compensated for cannot be degraded.

The lambda probe is not responsible for the demand for less CO₂ emissions for climate protection. However, the lambda control prevents one possibility, as indicated in the diagram above, namely to shift the mixing ratio somewhat in the direction of "lean" (λ > 1). Because then the nasty nitrogen oxides gain the upper hand and have to be fished out much more laboriously than with the three-way catalytic converter.

Here you should get an impression of how this described Nernst probe, named after the inventor of the principle, looks like when you remove the air-permeable protective sheet metal jacket. There's just something like a tall hat made out of ceramic, zirconium dioxide to be exact. In our picture, the heating rod is also drawn in.

What is this zirconium dioxide supposed to do? It has the basic property of allowing negatively charged oxygen atoms, known as oxygen ions, to pass through. That is the real reason why a lambda probe can also be called a power generator to a modest extent. In any case, it is enough for a multimeter or the evaluation by the engine control unit.

But why do the oxygen ions make the arduous journey through the ceramic? This is pressurized from the outside with the exhaust gas from the engine and from the inside with normal outside air. Now all that is missing is to mention the electrode layer inside and outside on the zirconium dioxide and the difference in oxygen atoms inside and outside causes a voltage difference.

Now place one layer of electrodes on the housing of the probe or the ground of the heater and lead the other outside, then you can pick up the respective voltage there. Since the other design that still can be found, the planar probe, essentially corresponds to that of a broadband probe, we will deal with it in the next chapter.

The resistance jump probe, which is much less common, dispenses with the reference from outside air, but uses the change in resistance with different composition of the mixture. Since resistance is always measured using the voltage drop, in this case with a supply voltage of 5V, there is a much higher resolution with an ideal value roughly in the middle.

So where are to find two-point probes? First of all, exactly where the exhaust gases of all cylinders come together (picture above). If these are too far apart, e.g. in a V-engine, then you just take two (picture below), always considering the best possible utilization of the engine temperature.

Originally, for example, with the transition from central to cylinder-selective injection, the need to use a probe per cylinder was seen, but that has not been realized. One can assume that the change in the values output by the probe occurs so quickly that a control unit learns very precisely which cylinder is currently responsible for a slight enrichment, for example.

Here is still a comment on the control technology. Varying speed at which individual components of a control loop operate is quite important. The delay caused by the effect of a changed injection signal on the combustion, its exhaust gases or composition and measurement thereof is typical. Here, the much faster electronics have to be slowed down artificially, otherwise over-regulation occurs.