2016 BlueTec NOx sensor issue
 

2016 BlueTec 4MATIC E250, took delivery on Nov. 2015. Did DIY engine oil change on Nov. 2016 using Valvoline 5W-40 MST.

Had a first CEL with fault codes P229F, P2201 in Aug. 2017 at 6.4K miles. Took to dealer, replaced both NOx sensor under warranty. Last week, had the CEL with the same fault codes again, now at about 7.4K miles.

Plan to take to the dealer again later this week, but how can a replaced NOx sensor went bad in just 2 months/1K miles? I wonder if this sensor issue is masking some root problems. I don't want the dealer to just replace the sensors again and call it done. Any suggestion on what might be the root cause? What direction shall I point the dealers to fix the root cause?

NOx sensors seem to be a weak link on a lot of modern diesels, from what I've read.

OK, these sensors are like $300 each, and i believe i have 4 of them with my 4MATIC. Sounds like an expensive repair down the road after the warranty.

The good (and bad) news is that the quality is spotty. Sometimes you get a good one. Indirectly related, I put 200K miles on a 2014 diesel (not MB) and it's still on its original NOx sensors, so it's not entirely hopeless.