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.