Nox Sensor yet again!

[Uky]

What happends is that when starting the engine, there is a lot of condensation forming in the exhaust system that gathers
in the lower parts of the rear mufflers. When the driver then uses the brakes, the (cold) water rushes forward where it hits
the (warm) NOx-sensors resulting in sensor failure.

Following the failure of a lot of NOx-sensors, Mercedes released a service information sheet: Small holes (~3mm) should be
drilled at the lowest point of the rear mufflers. This will make condensated water to get out safely, preventing it from
reaching the NOx-sensors.

I did that myself on my C238. Watching outpouring water forming a large puddles on the garage floor was quite an experience...

 

[rayaans]

I dont think NOX sensors going has anything to do with short journeys.
I have a petrol which you would have thought is perfect for shorter journeys but the nox sensor still went on it - albeit after the car was 8 years old.

 

[OneForTheRoad]

I can't see how water can reach the nox sensors , seeing as the dpf is uphill on most . I've read of condensation forming on the sensor tips , but not from water gushing rearward from the mufflers .

 

[TC350]

Having had NOx sensors replaced three times on my petrol C350, I’m getting convinced that they’re not actually failing!

I say this because I last had them replaced about five years ago. Since then I’ve adopted a different approach. I clear the fault code with my cheap OBD2 scanner, wait a week or two for the code to pop up its ugly head again, clear it again, then repeat. After clearing two or three times, it gives up and goes away! A year or so later, I need to go through the same process again. Probably takes me all of five minutes altogether.

If the NOx sensors are really failing, how come that my code clearing process lasts for so long? If it’s genuinely failed, surely the code will pop up again almost immediately? I suspect that it’s no more than a case of the software algorithm used to determine that the NOx levels are within a set range is just a fraction tighter than necessary. So anything less than optimum conditions flags up as a fault, which is translated as a sensor failure.

I have nothing to confirm this theory of mine, but NOx sensor manufacturers could well be making a fortune when there’s nothing wrong with their devices. Reminiscent perhaps of the old tale of train wheels having to be replaced after ‘not ringing true’ when tapped with a hammer. It took several wheel replacements before discovering that the wheeltapper’s hammer was cracked.