Emissions history:
5/18/21 62K miles - Emissions Modifications completed by dealer
8/23/24 87K miles - upstream temp sensor on exhaust replaced (under warranty)
10/20/25 97K Miles - CEL light came on, then 2 days later.... after parking in my garage, I smelled a suffocating ammonia-like odor coming from tailpipes.
Brought to local dealership who performed the emissions mods. Their diagnosis, after 3 days, was a faulty adblue temp sensor inside the DEF tank. Part ordered, took another 5 days before got call that it was ready to go. (Had new loaner vehicle during this time)
They said it was covered under the extended warranty
Drove home OK, Then when I went to work, within 2 miles on the highway engine light came back on, and potent adblue smell (and faint steam-like exhaust) coming again from exhaust.
I'm guessing they didn't get this up to full running temperature to test the emissions system properly? At any rate, it's back at the dealership now and will see what they find this time around. Either the injector is stuck open and letting too much DEF fluid to be released, or another faulty sensor causing it to. We'll see...

Lethal?

All exhaust is lethal if you breath enough of it.

I re read your post - you didn't mean someone died - which lethal suggests.

you probably wanted to use the word noxious or something like that.

got it.

Sorry for your troubles

Ammonia can be lethal. It was a big problem in the early days of refrigeration and air conditioning.

Your lucky !! You’re under warranty! Just think if it wasn’t covered . Yea the diesel motors are great, the pollution controls ruined them . I would not buy one because of what you’re going through . It’s very common . I’m sure resale value proves that .

Totally different circumstance.

FWIW
Anhydrous ammonia (NH3) has a 30 minute escape time at 300 PPM (0.03%) without lasting health effects. Dangerous because it reacts with moisture in the lungs causing tissue damage.

Ammonium hydroxide vapor NH4OH which is what we normally think of as the ammonia smell can but seldom reaches dangerous concentration in air.

8 hour workplace exposure limits (US)

NH3 - 25 PPM odor is unreliable for detection of NH3

NH4OH - 50 PPM which is also the odor threshold

I found the "headline" to be a bit dramatic.

Falling into a large vat of moonshine can kill you. Being born, it's my understanding, can ultimately lead to death as "parts" wear out or get plugged up.

Admittedly it was a bit dramatic... but made for some interesting replies. Choking, suffocating, like opening a bottle of ammonia and taking a good inhale - that's what it was like.

Other than the one temp sensor, it's been problem free and super reliable up to now since purchased in 2020.

Quit being a Karen...Peter..... ;-)

The ammonia smell, being repulsive and an irritant to the lungs, is a good thing as it alerts you to a problem/situation unlike the "silent" killer, carbon monoxide. Ammonia-based "smelling salts" are used to bring people back to consciousness and not to kill them although I'm sure, in a sufficient concentration, it could kill.

You should change your username to Fritz Haber!!!! :-) :-) :-)

The Witness Protection Program will not allow me to change my name.

The thread title definitely is descriptive.

Well, I died reading this

Haven't heard a peep yet from dealership; not to be dramatic or anything, but I hope all the diesel mechanics are OK. Rumor has it the entire service department has been put under quarantine, while they await an emergency response team to arrive from Stuttgart to assess and remediate the situation. I fear my car will forever be labeled as 'vehicle zero' and have zero remaining value. Maybe they will never reopen, and I'll be able to keep the brand new GLA 250 'courtesy vehicle' indefinitely; not a bad ride!

Meh, it was the Chicken Salad....I will go out and fire up my diesel in your honor.....(I do need to find a slow oil leak that I am certain I created)...maybe I should dump the black water tank onto my driveway in your honor....

Update: dealership replaced both upstream and downstream NOx sensors. I still smell a little bit of the exhaust odor, but not anything like it was, didn't knock me over

I see you’re from CT. Hopefully the dealer that did the work isn’t the one in North Haven. They are thieves. My car was in for warranty subframe replacement and they attempted to charge me labor to R&R the springs and exhaust. Those things have to come out as part of the warranty work.

Nope not going there. Went for trip yesterday (15 miles each way), and on way back engine light came back on, but only a mild smell of ammonia; have to bring it back again, the drama continues

Wait, the genius squad at the Stealership did not get this clearly earth destroying issue with your atmosphere killing - selfish mode of transportation? I will report them immediately to crconsulting for retraining to get up to that expert status.........never, I mean never trust anyone but a "Mercedes Dealership" mechanic.

exactly..... seems like they are playing wack-a-sensor mole, not really getting to the source of the issue, which is lurking beneath -

Finally have car back after 10 days at dealership..... this go around the SCR was replaced, along with upstream NOx sensor (which was already replaced on previous stayover). They kept it for awhile, drove it some 80 miles in all to be sure CEL didn't pop up again.