Firstly, my car is a 2016 model so is well out of warranty.

In July my EML activated and after getting in plugged in at a local garage it indicated a NOx sensor fault. After checking my car for any outstanding recalls there was one for a diesel software upgrade, I was hoping it might be linked to the NOx sensor fault so, begrudgingly, I took it to Mercedes who then diagnosed that the upstream NOx sensor had failed and they were unable to do it on goodwill as it’s caused by water ingres/corrosion, they couldn’t do it straight away either because the NOx sensors were backlogged so I had to wait about three weeks. Eventually came in and replaced at a cost of around nearly £700. They also did the software upgrade too.

Not had a problem since then, until around three weeks ago. My AdBlue tank flashed up that it was low and needed topping up, had some in the garage so I put the full 5L bottle I had into the tank. No sweat. Two days later, my EML activated again. Before I had time to take it to a local garage to get the fault code read due to it being a weekend and having work etc, my AdBlue tank flashed up on my dash indicating that it was ‘unable to measure liquid level in AdBlue tank’ and that the engine would shutdown it ~500 miles because obviously it thinks the tank is empty as it cannot measure the level.

I got another appointment at Mercedes and they’ve just rang back today. They are saying it’s a NOx sensor again, caused by water ingress. They said that they recommend a water test to see where the water is coming from (which would be free of charge) however I’d have to pay £190 for them to strip the entire interior which would be around 4 hours work.

What are my options here? Shouldn’t they have advised this after they said the last one had water ingress? I was expecting a full AdBlue tank replacement but this seems worse due to the fact it could have been avoided if they advised a water test last time? I’ve heard people getting NOx sensors replaced FOC but I feel like I’m just getting reamed.

Thanks for reading.

 

Hi, take a look at the document I attached, it's a Xentry TIPS document explaining what happens when the condensation drain hose from the climate control is not installed correctly and it causes the NOx sensor connector to oxidize. I think that it's possible to repair the connector of the sensor without replacing the whole NOx sensor, but I'm not an expert.
These NOx sensors fail fairly often, I have a 220d 2015 and it had a sensor replaced in 2017, then both in 2019 and than again one failed in 2021. When they replaced the last sensor they also did the software update and added the SCN code 30O to the datacard of the car. 30O is supposed to be updated software for a revised NOx sensor which should not fail that often.
If the sensor you replaced failed again within a year, I think it should be covered under warranty and Mercedes should replace it free of charge (My service advisor told me that they give a one year warranty on those sensor).

I also heard a lot of people getting nox sensors replaced FOC but Mercedes told me last year that it only applies to cars less than 5 years old. In the US the warranty/goodwill was extended to 10 years on emission parts because of a law suit. https://mbworld.org/forums/diesel-fo...-warranty.html

 

What make of Adblue did you use ? My 2017 does NOT like Redex (bought from Costco) and kept throwing up the EML. Since I started using Halfords stuff its been 100% fine.

Dont get me started on the massive increase in cost for Adblue

 

Quite a few of the Shell garages (at the very least the ones with HGV pumps) have AdBlue pumps, I always found it way way cheaper with prices around 70p per litre and it worked just fine on my C220 back when I had it.

 

Thanks, I need to find a local(ish) one of these - if only to save on the plastic waste of the 10L container

EDIT - found an app 'AddBlue4u' which has located some reasonably local ones - happy days

 

It sounds frustrating dealing with recurring issues. Since water ingress caused the first NOx sensor failure, Mercedes should have recommended a water test earlier to prevent this. You can ask them why it wasn't suggested during the first repair. You should also inquire about the possibility of a goodwill repair, especially since it's a recurring problem. If you’re not comfortable with the £190 charge, consider getting a second opinion from an independent mechanic. If the issue is related to a design flaw, you could escalate it to Mercedes customer service for further support.