Fixing the “Auxiliary Battery Malfunction” Warning in a Mercedes.

If you’ve owned a late model Mercedes any length of time there’s a good chance you’ve seen this message. It pops up on the driver information panel in the dashboard and can seem rather alarming. The good news is, while you should get it fixed, it’s not something to worry about unduly.

In this post we’ll explain the auxiliary battery replacement process in a late model GLE350. It’s not a major Mercedes repair, although there is a little work involved. Here’s what you need to know.

What is the Auxiliary Battery?

Every car, SUV and truck has a 12 volt battery for starting the engine. Since the mid-2000’s Mercedes has also installed a second battery. This is what they call the auxiliary battery and it’s just a smaller version of the main battery. (To be accurate, in a few Mercedes-Benz models it’s actually a capacitor, which is another device for storing electricity.)

The auxiliary battery is a backup for the main battery. It does a number of jobs depending on what model of car it’s installed in. These range from powering electrical accessories like the radio/infotainment to operating the engine stop-start system.

Like the main battery, the auxiliary battery is charged when the engine is running. And just like that battery, it lasts around four to six years. When it can’t hold a charge the computer pops up that malfunction message.

How Serious is it?

The simple answer is, not very. When the warning comes on you probably won’t notice any difference in how the car drives. (In some models drivers report that the engine stop-start stops. You might consider that a good thing!)

However, the auxiliary battery is there for a reason. Even if that reason is to be a back-up you still shouldn’t ignore the warning. Doing so could cause some electrical issues, which are probably best avoided.

Auxiliary Battery Replacement in a Mercedes-Benz GLE 350


Fixing the “Auxiliary Battery Malfunction” Warning in a Mercedes with EurAuto Shop in Plano Tx. image of area in mercedes where the aux battery is found, under the 2nd row seating

If you plan on doing this job yourself you’ll need a good set of Torx tools and you’ll need to know where Mercedes hid the battery.

In the 2012-2019 GLE they put it under the rear seat on the left side. In other models it can be behind the carpet on one side of the trunk, under one of the front seats or under the glovebox.

There are several different types of auxiliary battery so make sure you buy the right one!

In the GLE the first step is to flip the seat squab up and forwards. That uncovers the bolts holding the seat back and the seat belt latches in place. Behind the seat back there are more bolts that need removing. These are under the carpet so to get at them you’ll need to remove the cargo loops that are bolted to the body.

With the seat back out the way you can lift up the carpet to get at the battery. Swapping them over is straightforward, after which you reinstall the seat back. It is very important all the bolts are tightened correctly, but not overtightened. Seat backs and seat belt latches are safety-critical items!

Last, you can’t just put these batteries in the trash. Make sure to dispose of the old battery properly.

Get Mercedes Repair Work Done at EurAuto Shop in Plano TX.

We specialize in service and repair of high-end and exotic European vehicles. When you entrust your car to us we’ll give it the care and respect it deserves. Make an appointment today or call (214) 552-4194 to discuss your automotive needs.