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2016 GLA 250, FWD, 2nd Owner, ~28k miles
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Discussion Starter · #1 ·
Hello GLA Owners:
Bear with me as this post will be a bit detailed, but I always err on that side when it comes to these kinds of things since I feel more is better.

I am from Miami, FL, have a 2016 GLA 250 (FWD) with 28k miles and had the Aux battery and Wheel Speed sensor replaced this past Spring. Prior to that I had the Aux Battery Warning + all the failed driver assist functions + parking brake, plus Brake HOLD, plus power steering become inoperative. Those repairs appeared to fix the issue.

About a month ago, and not many miles after the repairs (I barely use the car since I work remotely now), I had a situation where the driver assist functions and power steering, etc. all failed when pulling out of my garage, but this time without the Aux Battery Warning.

After sitting in <Park> and idling in the parking spot for several minutes was the first time I saw the "Reverse Not Possible" warning. Went in for a doctor's appointment, came back out, and was able to get it back into reverse, but the driver assist features + power steering, etc. were all still inoperable.

A few minutes later, while driving home, all those systems went back online, it was immediately noticeable when the power steering comes back, so my assumption was that the new Aux battery is draining from limited use, and the driving I had done charged it back up to where those systems could function.

HOWEVER, would a new but drained Aux battery cause the "Reverse Not Possible" issue?

AND, would not driving the car for a week or two at a time cause the Aux battery to drain that much?

Last night's occurrence, however, gives me pause. I actually had the systems do their shut down again last night. BUT, I heard a sound that I hadn't noticed the previous times. It sounded like a "pump" humming, very low and dull, almost like an A/C compressor, but not the A/C. I noted that I hadn't heard that before, then when pulling out of the garage, all the systems shut down again. The difference this time, I drove for 2+ hours thinking that would definitely recharge the Aux battery, but unlike that more recent time where the systems restored while driving, the driving last night did not fix the issue. I had shut it down, wait, start it back up and the systems were fine until I started slowly pulling out.

And the coup de grace, Reverse Not Possible showed its ugly head again. Same as last time, after idling in <Park> for several minutes. It does seem that after sitting off for several minutes makes that go away, but why is it happening?

Unfortunately, I had a trip I couldn't delay or cancel that was away from home so I am nowhere near the dealership that did the work in the spring, not until Saturday at least, when I return home.

The car was bought with 21k miles on it in 2019, and added the 3 year extended warranty and it currently sits at 28k miles. The repairs this past spring... the wheel sensor was covered, the Aux battery was not. They also discovered the CV boot was torn and leaking, so I was on the hook to replace that, but better that then the entire front axle.

Thanks in advance for any info from others who have had this happen.
 

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The thing that rings out to me is working from home ... not driving much. I would start with a stress test on the main battery. Modern cars have sensors everywhere. Voltage matters. The battery is always my first concern for these kinds of problems.

Good luck and please report back when you make discoveries.
 

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2016 GLA 250, FWD, 2nd Owner, ~28k miles
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Discussion Starter · #3 ·
That's my thought as well, just not starting it and driving it for weeks at a time sometimes.

And yes, once I figure it out, I will definitely return back to report. The thing that freaks me out the most is the not being able to reverse when that happens.
 

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2016 GLA 250, FWD, 2nd Owner, ~28k miles
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Discussion Starter · #4 ·
So, heard back from the dealer:

Another wheel sensor malfunction is the primary cause of the dash warnings. This was an issue last time that also had the Aux Battery Malfunction. The wheel sensor is covered under warranty.

However, they are also saying the main battery needs to be replaced, which makes sense since the car is 5 years old and they are saying that is the likely cause of the "reverse not possible" and I also experienced the car going into "neutral" while in drive on the trip home, but I will do that work myself and save half the amount they want to charge.

Whether that seems to do the trick, I will report back here on those two issues.
 

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If you do end up getting the new primary battery (mine started having issues after just 2-3 yrs, think prior owner drained it bad), make sure with not driving much that you get a good battery maintainer. I drive my car 1-2 times a week, small drives usually and every 6 months or so I will plug it into the maintainer just to check if it quickly charges, so I know I have not allowed it to drain much.

I did not have the reverse issue you did. As I recall, as it got colder out I simply could not leave the car (in garage) more than a couple days without starting it otherwise the battery was barely able to start it.
 

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2016 GLA 250, FWD, 2nd Owner, ~28k miles
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Discussion Starter · #6 ·
If you do end up getting the new primary battery (mine started having issues after just 2-3 yrs, think prior owner drained it bad), make sure with not driving much that you get a good battery maintainer. I drive my car 1-2 times a week, small drives usually and every 6 months or so I will plug it into the maintainer just to check if it quickly charges, so I know I have not allowed it to drain much.

I did not have the reverse issue you did. As I recall, as it got colder out I simply could not leave the car (in garage) more than a couple days without starting it otherwise the battery was barely able to start it.
Good idea, thanks for the suggestion!

Didn't get the battery at the dealer, but will be getting a new primary one this week and will report back. So far, have not seen the "no-reverse" warning or experienced the suddenly into neutral since getting the wheel sensor replaced.
 
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