2016 C300 4Matic
I went outside a few days ago and the trunk was open. I think I had accidentally hit the trunk open button on my keys.
Apparently, the battery got drained. I went to start it about two days later and the battery was was completely dead. I attached it to a trickle charger an let it charge for a few hours.
Everything turned on, however the engine would not crank when I attempted to start it. I took it to the local auto parts, they put it on their charger and made sure it was fully charged and tested it, which it passed.
I reinstalled the battery, same problem, electronics turn on but the engine wont crank.
I am getting a traction control inoperative warning, power steering malfunction, and service required warning. why would these be turning on from a dead battery?

Today I pulled the starter fuse and starter relay. Tested both and they are both fine.
So I'm not sure what to look for next. any ideas?

 

I would also check the aux battery as it is possible that, because of the starter battery got discharged, it strained the aux battery.

 

Thanks for the tip. I replaced the AUX battery, still no crank no start...

I remembered I had an OBD reader that connects to my phone via Bluetooth. Here is what it says:

 

Hmmm, I wonder if for any reason it thinks something happened (ie. collision) and disabled start, if so need to renable start through Xentry or a very specific third party scanner that deals with Mercedes. I forgot which one has it but I remember seeing it. Xentry is not a must if the scanner has the feature/function. What you could try is erasing those pending codes and see if it does anything in the meantime.

 

So quick update. Had to take it to the dealership. They found some kind of wiring issue to the electronic steering. I'm trying to get more details right now, but the ESL wasn't able to communicate with other modules.

Now they're telling me that my brand new battery is no good, and they want to charge me $650 to replace the battery and "calibrate" it. Has anyone heard of this before? Sounds like dealership scamming to me.

 

Here is the problem. When your battery died the modules all reset however when you charge the battery, the modules didn't like the battery for some reason as they need full power after module reset. How old is the battery? I have noted that AGM look like they have a full charge after discharge but that does not seem to be the real case to the car. I would go an purchase a new AGM and install it and your problems should resolve.

 

So I got a final resolution on this. Turns out the neighborhood rabbits chewed on a wiring harness connected to the power steering under the car. The battery drainage was possibly a symptom of this. Once the wiring was repaired & rewrapped, the car started up with no problems and no codes.
They also told me that the battery needed replacement, as I put in the other thread. I politely declined, and sure enough, when I got the the dealership, the car started up just fine and the battery was just fine.

 

Wow.. now that is something only a dealer could find.

 

I have a GLC300 2017 Base model. For others who may be wanting to replace the battery here is what I went through yesterday.

Original battery had been weak for about a year (lasted 7+ years, including 2 years of not driving much in Covid) - giving me the "Stop vehicle, Leave Engine running" message on dash. My local non-dealer-Merc-specialist shop had confirmed that battery was about 65% life but it took me a while to change it. Car was still starting fine but I thought it was time to change and get rid of the annoying message that can't even be dismissed.

After research on this forum and internet, bought a H6 for $170 + tax from Costco (850Ah). Hooked up car to the battery of another car to keep settings. Removed the dying battery and put in the Costco one. Disconnected the other battery. When I tried to start the car it gave me all the same errors the pictures above. Car would not start.

Finally, after trying various things with no luck and deciding to get it towed to the shop, I just disconnected the newly installed battery (removed negative). Left it alone for 2 min. Reconnected. Car started up with no issues. Has been running fine for 2 days. Other than the seat belt tensioner setting and 1 or 2 other minor ones there were no other issues or settings that were lost.

I went to my local Merc specialist shop and they told me that this model does not have anything to reset nor any training for the new battery to do. However, they also recommended I call the Merc dealer, which I did. They also stated that this model does not have any counter to reset nor does it have hardware that that stores changes to how the battery is charged as battery ages; hence nothing to reset here either.

Posting here because of all the contrary info on this topic on the internet and no clear direction.