An observation that may save some time, frustration, money, and inconvenience.
2016 Base Model. I bought the car with 4000 miles and one year old. Factory maintenance as required and exclusively dealer maintained since.
Six month oil changes and battery checks since Florida batteries rarely last more than three years.
This car has 41000 miles on it today and battery maintainer for weeks/months at a time, under cover in the garage.
After about three years ownership the Auto Stop/Start quit. Out of warranty, the dealer asked if i could live without as it was $1000 plus fix. I lived without.
The back-up side mirror dip feature quit. Another thing I could live without.
Last year, at an inopportune time, the car wouldn't turn over. Had a new battery installed, and to my surprise the Auto Start/Stop started working. The mirror started dipping,
and all the courtesy lights I had not noticed started working also. Auto load shed?

Drive on..

 

Yes, you've discovered the well documented fact that these cars are very sensitive to battery voltage (lots of threads on here about it). Welcome to the forum!

 

Exactly that. My stop start hasn't worked for a couple of years - I know that if I put a brand new battery on it, it would. The cut-out voltage for the stop start to work is ludicrously high. Not that I particularly want it to work!

 

So that's why after replacing the AGM with a Lithium (antigravity H8) that my auto stop/start resumed working, and consistently every since! It has stopped operating for a good year before that. The Lithiums resting voltage is over 13V and slightly over 14V when running.

 

My auto-start stop worked a few times once I bought the car, then never again. Original battery. Same thing on my parents GLC43 when the battery got old, except they don't maintain the battery so it was simply replaced and resumed working.