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If you get this message and the car shuts off or will not start: turn the car off, wait for a few seconds, restart.
The reason you are getting this error message is because the secondary Aux battery in your Mercedes has died. This is not a major issue, just annoying as start/stop will not work. Drive the car in normal or sport mode so the car doesn't shut off.
Simple fix: go to Autozone and let them know the year and make.
The Aux battery is under the passenger carpet and under the glove box.
To access make sure you remove the cabin air filter cover.
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fyi
I took mine out 3-4 years ago and all good, just a message pops up every time I start the car saying 'aux battery malfunction.' Click the back button and it clears and doesn't come back until you restart the vehicle.
The aux battery supports start/stop and brake hold, both of which I could care less for.
 

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Mine had been replaced by the dealer before I took delivery of the car, so I wasn't very hard pressed to completely remove it. Also, it is possible that some sort of reset may be needed from Xentry.
Also, there surely is a procedure, in relation to what to do with the main battery, do you disconnect it before or after disconnecting the backup battery and what other things may need to happen.
Although I have the WIS for a GLA45, I have it only for the old setup, my Xentry is not that new anymore.
 

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Well, you may want to know that:

-Mercedes these days have an on-board communication network, called the CAN
-The CAN has network switches in the nodes called SAM, Signal acquisition modules
-These switches are expensive, even by Mercedes parts standards, unreasonably expensive
-In the W204 platform (my other AMG), the C63 is the only one not to have a secondary battery, all other W204 powertrains, including diesels have a secondary battery
-In this W204 platform, there is a very, very high rate of SAM failure coincidence with replacement of the secondary battery

Cliffnotes: You replace the secondary battery and BAM, the rear SAM is dead and you have to shell out $$$. It also needs to be programmed via Xentry, before the new one can function.

Hence the idea, in this case, to follow whatever they say to follow, to avoid killing one of the SAM's.
 

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Well, you may want to know that:

-Mercedes these days have an on-board communication network, called the CAN
-The CAN has network switches in the nodes called SAM, Signal acquisition modules
-These switches are expensive, even by Mercedes parts standards, unreasonably expensive
-In the W204 platform (my other AMG), the C63 is the only one not to have a secondary battery, all other W204 powertrains, including diesels have a secondary battery
-In this W204 platform, there is a very, very high rate of SAM failure coincidence with replacement of the secondary battery

Cliffnotes: You replace the secondary battery and BAM, the rear SAM is dead and you have to shell out $$$. It also needs to be programmed via Xentry, before the new one can function.

Hence the idea, in this case, to follow whatever they say to follow, to avoid killing one of the SAM's.
so, a couple years ago, I killed the whole system dead flat, sitting in the drive for a few weeks.

It was stupid hard to get a third battery and a schumacher charger inline with the 2 dead merc bats and get charging current flowing. took hours before I had enough juice in the car to remove the support Bat, overnite before it was free of codes and acting right

Ya think I should see about having them replace both before warranty is up? Eat the Merc Battery charges, but have the SAM issue covered should it happen
 

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If you get this message and the car shuts off or will not start: turn the car off, wait for a few seconds, restart.
The reason you are getting this error message is because the secondary Aux battery in your Mercedes has died. This is not a major issue, just annoying as start/stop will not work. Drive the car in normal or sport mode so the car doesn't shut off.
Simple fix: go to Autozone and let them know the year and make.
The Aux battery is under the passenger carpet and under the glove box.
To access make sure you remove the cabin air filter cover. View attachment 26648 View attachment 26649 View attachment 26650 View attachment 26651
Awesome, thanks. Assuming I'll need to be doing this within the next year.
 

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GLA45 (2015 - MY2016)
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Same! I have a 2017 GLA 250 4matic and need to replace the AUX battery (which i want to do myself rather than dealing with the dealership...) but no idea how to get to it. The little box unclicks and only halfway comes out. Any ideas?
This guy on You Tube shows you how (the tricks in removing the cabin air filter cover first which is just above).

I just changed my Aux battery as was getting the dashboard message - it fixed the 'Hold' brake from not working but Stop/Start is still not happening (will move onto main battery next to see if that's fixes it).
 

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This was not found, but to recap, it is something from a different and older platform, the W204.

Yesterday I was checking my GLA and was looking for available check-ups and procedures relating to batteries on the Xentry. I got only as far as battery sensor before the lap top battery went low.

In the process, there was a stored code on my car about:
communication between sunroof module and air conditioning controller.
In the process I bumped into another feature shown on the Xentry:
Trailer detection.



Yeh, everything is connected in this car, everything reacts.
There was another potential "feature" incident that I am investigating.
The car was stopped and windows down by 1".
The key was in my pocket.
I was standing next to the car chatting with a coworker. As the conversation extended, I leaned against the car. The window proceeded to lower itself by an additional 2-3". I got startled and stepped away from the car. The window went back up.
The guy asked my if my car is sentient :)

I think it's probably as intelligent as a not-too-bright dog. Maybe more...
 
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