The owner of BMW approached me with an extraordinary story. He installed a new NOx sensor, but after 400km, the error message 2AF8 appeared.
This error message means – the NOx sensor has indicated a reduced Lambda value in Overrun mode. The typical cause of this error message – clogged probe of the NOx sensor. Unfortunately, it happens. There can be several reasons – increased oil consumption of the engine, some fuel additives, water in the exhaust, and some mechanical contamination. However, not so fast – during 400km!
The owner of the vehicle performed a test of data of the NOx sensor in Overrun mode.
Lambda linear still is 1.0 (although it should be at least 15 .. 20+), Lambda binary: 544mV. Yes, obviously, the probe is completely clogged! Or – it has lost any performance abilities.
I suggested dismantling the sensor and perform a visual inspection of it.
The mechanic dismantled the sensor and reported that the signs of thermal(?) damage to the wires had been detected. Here, some images:
The mechanic’s statement – the sensor is clearly damaged, its wires started to burn!
The case is extreme, so it requires a serious investigation and analysis. Burning wire can cause the burning down of the whole car, so such cases need serious attention!
The situation mentioned by the mechanic sounds strange. I will mention only some arguments, which overturn this assumption.
a. the wires of the sensor and their insulation tube are intended for work temperatures till 800oC. To cause permanent damage to the wires, the temperature should reach several thousands of oC, which means – their conductor (metal) should start to melt. There are no melting signs thou. Additionally, if the temperature would have reached several thousand of oC, there should be signs of burning in other parts of the vehicle too;
b. The cross-sectional area of the wires is 0.75mm2, the power supply to the sensor is supplied via fuse 06 with a nominal 10A. The wires are not long (their length is 70cm), heat capacity via wires can not reach such high values for wires to start to burn. In addition, power wires to the NOx sensor itself are not thicker (0.75mm2); they are also much longer – actually, they first would have to start to melt;
c. the NOx sensor does not has such “powerful” paths to withstand current with the strength of 10 .. 15+A. In case of a complete short-circuit, the appropriate wire to PCB of the sensor would burn;
d. have to assume that there would be a short-circuit in the probe because, in a normal situation, the current via the heating element of the probe is only around 1.0A, which can not cause any damage to the wires;
e. for example, the black and yellow wires are the probe signal wires, the current, which flows via them: several mA. But, as you see, exactly the black wire has suffered the largest damage.
I believe, it becomes quite clear, that the wires didn’t burn due to damage of the sensor. Let’s see the next possible scenario – the wires have burned due to the impact of the extremely high outside temperature.
In theory, it is possible that the sensor wires:
a. have touched the exhaust. It is possible that the temperature of the exhaust (due to incorrect fuel mixture) has overreached 700 .. 800oC;
b. has touched drive shaft – in such case, the very high temperature could arise due to extreme rubbing.
To check how the wire and their insulation behave in the impact of high outside temperature, I subjected them to the extreme test. Here, the video, how I try to burn the insulation tube of the wires:
As you see, the wire insulation tube even starts to burn, but even burning it for a longer time and shaking off the ashes, you see that the insulation “frame” has not been burned:
It looks completely different than in pictures sent by the mechanic!
Then I tried to burn the wires:
The burning of the wires was unsuccessful! Even after longer burning, cleaning off the soot, it turned out that the insulation of the wires was not damaged at all!
My conclusion is clear – the defect is NOT thermal!
Obviously, the defect is of mechanical nature. I took the file and damaged the wire (the white one) mechanically. Here, what I got:
And here, two images, sent by the mechanic – I zoomed in the places of the damage to make it visible more clearly:
In my opinion, there is no slightest doubt that wires were damaged mechanically! They are compresses, scratched.
How can such a situation happen? Obviously:
a. the mechanic has not properly secured the wires/placed them below the decorative cover;
b. the user of the car has hit some hurdle, which has damaged the wires. If the wires were properly secured, such damage would be impossible. Here, the unprofessional “performance” of the mechanic has made losses of several hundred EUR during one moment. In addition, I believe that the mechanic saw the consequences of his “performance” when demounting the damaged sensor and understood very well that these damages were created by his own mistake. But – decided to hide it and lied to the owner of the car. This will remain on his conscience. Of course, it is also possible that the owner of the car (or the user of the car) also decided to keep silent regarding the unsuccessful maneuver to avoid unexpected costs.
The damaged wires (creating the short-circuit between them) have created permanent damages to the electronics of the sensor and probe. Unfortunately, it is not possible to save such a damaged sensor.
Do not entrust the repair of your car to non-professionals!
