Replacing Lambda probes

In this entry, I will mention some specific nuances, which have to be taken into account, when replacing Lambda probes (also NOx sensors). True thou, if I will count them before explanation, you will think, that these requirements are not only exaggerated but also non-logical. That’s why let’s start with my own experience!


My experience in work with Lambda probes.

In my practice, I often use Lambda probes. Most probably, it’s more often seen component from automotive tools. One day I found, that the wire of my “every day” test probe is dirty. It was soiled with sand, oil. For cleaning, I used the first cleaning agent available – Cillit. I cleaned the wire in several minutes, dried it out, turned on the probe to test it and… it didn’t work anymore!

When measuring the air in the atmosphere, probe didn’t displayed Lambda 15 .. 25 (depending on oxygen/CO concentration in the air), but Lambda around 2 .. 3! The situation didn’t become better after repeated cooling down and heating up, also – blowing the hot air in it. The probe has lost its ability to “sense” the oxygen. But I did everything so carefully. On the body of the probe (not even talking about getting inside), didn’t gets no water, no cleaning agent!

I checked the management electronics, connected other probe – there was no doubt, that my test probe is damaged.

The most interesting part started on the next day – my test probe “got better” slowly!


I already forgot this strange case, but after a while, the same happened to another probe. And then one more, which “got sick”, after I cleaned off the paper label of its wire with nail polish remover. Yes, after removing the label, all room was stinking, the probe “lived” in this environment for several hours. On the next day it was totally damaged: instead of Lambda 15 .. 25 it indicated around 1.5! During 10 .. 15 minutes in the working temperature (around 800 oC) Lambda increased to 3. Heating the probe for one more hour, Lambda reached 5. After half an hour: it reached 7. For two more motor hours, the probe restored the ability to measure rich mixture. Very slowly the probe came back to life. Unfortunately, Lambda by 8 the “healing” process stopped. To trim the probe for correct work, it’s calibration resistor had to be increased from 95 Ohm to 140 Ohm.

Yes, the probe still was in the range of technical parameters, indicated by the manufacturer, but without a doubt – such poisoning has done a permanent damage! Additionally, we have to take in account – such re-trimming of the probe is not possible if it’s used in “standard” applications (it means, without custom management system, which allows re-calibrating the probe). Using a probe in such condition (damaged: with modified sensitivity) – DME will record the error message regarding its signal, probably – will turn off the closed loop system of fuel regulation, and problems with the performance of the engine will appear very soon.


Counting at least 3 cases, when the probes “got poisoned” and lately at least partly “got better” again, I had no doubt, that this is not a mystical coincidence. Exact chemistry/physics of this process is not known for me, but first/more simple explanation, which comes in mind – if in these cleaning agents the oxidants are included, their molecules, when became in the active environment of the probe, could “catch” the oxygen ions, which the probe has to measure.


To make sure, that my assumption is correct, I performed an experiment. I poured two drops of nail polish remover on the cotton swab and brought it close to the Lambda probe.

As soon the cotton swab was brought close to the Lambda probe, it’s indications immediately displayed much lower content of the oxygen. During several seconds the Lambda probe indicated a rich mixture! At this moment I had no doubt, that the cause of “damages” is clear.


This phenomena also explains information, received for several times, regarding cases, when directly after installation the indications of Lambda probes were incorrect, but lately the probes “restored”.


Unfortunately, in case of heavy “poisoning”, the probe is damaged irreversibly!

Note: NOx sensors are even more sensitive than Lambda probes to any chemical and mechanical influence, they have to be installed even more carefully!


I think it’s time to mention some nuances regarding replacing the Lambda probes.

  • treat the probe carefully. Don’t drop it! This, maybe, sound’s stupid, but I’ve seen, how the mechanic drops the probe to the concrete floor from a height of around one meter, because the drill thread had to be cleaned, but there was no space, where to put the probe. The active element of the probe is ceramic tile, which thickness is 1 .. 2mm, and it’s very fragile!
  • During the installation of the probe don’t use any cleaning/oiling agents of spray type (brake cleaner, WD 40, etc). If necessary, oil the drill thread (to restore the thread), use a smear, which is especially intended for the exhaust systems.
  • If you still use a spray, while doing it, put the protective cap on the probe (included in the set). Before installing the probe, dry and clean with pressed air, before it will be installed, ventilate the exhaust system. Only after these procedures take off the protective cap.
  • If during the installation the sprays are used, keep in mind, that during first motor hours, the indications of probes can be incorrect. Accordingly – after this period perform the re-adaptation of the engine.
  • If the probe was used before, don’t use any sprays to clean it in no way! Clean the air vents of the probe with a dry napkin, blow with compressed air (not exceeding the pressure of several bars).
  • The exhaust system and connection of the probe have to be airtight! Even tiny air supply, even in connection “after” the probe can be the cause of incorrect indications of the probe and can cause the error messages regarding signal of the probes and also the performance of the engine (fuel mixture, trim of probes, etc).
  • During installation (applies also to NOx sensors) avoid to wear clothes from synthetic fibers; disconnect the car from all other electrical sources (charging, welding equipment, etc.). At first, the static voltage of the probe has to be reduced: keep the probe in hand, with another hand touch the exhaust system of the car (not, for example, if working in synthetic clothing, directly, without zeroing the potential, plug the probe in). If possible, use the antistatic bracelet – it will protect from damages of DME/DDE control circuits of the probes, serial interfaces of management modules and  NOx sensors.

Not observing security measures, which avoid the static charge, we can save several tens of seconds, but get in some serious trouble. If the chipset of serial interface of DDE/DME is damaged, you will have to open the unit, the repair will not be quick or cheap (and only several companies will be able to perform it). If the serial interface of the NOx sensor will be damaged, unfortunately, the sensor will have to be replaced (it is quite expensive – as we know, BMW OEM NOx sensor costs around 400 .. 500 EUR).

If the input circuit/chipset of the Lambda probe of DDE/DME will be damaged, the repair can turn in replacement of the unit. Unfortunately, CJ110, CJ120, CJ125, and similar chipsets, which are used in these units, are not available in open market, additionally, for example, CJ120, CJ125 have many releases (with different placement of control registers and their content), which makes the replacement of these chipsets practically impossible.


Not for no reason, each Lambda probe in the package closed with a special airtight cover.


Packaging of NOx sensor is even more serious!


When opening the package – the NOx sensor is packed in the airtight and antistatic pouch.


And finally – NOx sensor is closed with an airtight cover.


Here, the note from Denso, what should not be done.