UniEnergy Technologies Battery » How Do I Fix My Mercedes Auxiliary Battery Malfunction? (Causes& Instructions)

How Do I Fix My Mercedes Auxiliary Battery Malfunction? (Causes& Instructions)

No one likes to see an error pop up on their car’s dashboard, especially when it warns of malfunctions or electrical errors.

The Mercedes auxiliary battery malfunction usually pops up when the battery dies, and you can fix it with a simple replacement. While this is the issue 9 times out of 10, the other 10 percent gets frustrating fast.

In this article we explain what the malfunction pertains to, how to diagnose a bad aux battery, and how to swap it out (for all locations). We’ll also touch on what you can do if the problem continues after installing your new battery.

What Does the Mercedes-Benz Auxiliary Battery Malfunction Mean?

Your car has two batteries: the main battery (G1) and the auxiliary battery (G1/7).

The main battery, also known as your system battery or starter battery, starts your engine and serves as the main power source for all electrical components in your vehicle.

The auxiliary battery, also known as the secondary battery or backup battery, is usually an AGM battery that provides power if the main battery is low on juice. It may also power electrical accessories, such as radio or infotainment features.

The malfunction warning message lets you know that there’s an issue with the battery and it will not be able to do its job. Most of the related issues relate back to the battery itself, such as failure to hold a charge, but there are a few connected components that can fail and trigger the error message.

Why the Auxiliary Battery is Important?

While losing out on your entertainment system may not seem like a big deal, the auxiliary battery is a major safety feature that provides backup in case your main battery dies.

You’ll notice issues with useful features like:

  • Blind spot assist
  • Lane keeping assist
  • Radars
  • Stop/start functions

In the worst case scenario, you don’t have a backup to operate the electronically controlled gear shift selector in your car if your main battery dies as well. You won’t be able to switch gears, and cannot safely put your vehicle in park.

Symptoms of a Bad Aux Battery

Symptoms of a Bad Aux Battery

Even before your auxiliary battery malfunction error pops up, you’ll notice issues with your electrical system. This usually starts with accessories powered by the auxiliary battery, like heated seats, but it may not be significant enough to merit a second glance.

Just keep in mind that if your main battery dies, your car will not start. If your auxiliary battery dies, your car should start and pop the error code up on the dash, but that might be the extent of it.

Why This Happens

Your auxiliary battery charges when your engine runs, and it only lasts a certain number of years before it can’t hold that charge anymore. This means that even if you don’t use it, your battery needs replacement on a certain schedule.

This also happens when you have issues with connected components and the connection with the battery is severed or inhibited. The most common culprit here is the capacitor, but this happens less often than the battery simply aging out.

Prep Work for Auxiliary Battery Replacement

Once you get this error code:

  • Determine whether you want to fix the problem on your own or have a mechanic address the issue (the latter choice can stop here)
  • Determine where your auxiliary battery is
  • Gather the necessary tools for the job

These steps remain the same regardless of what model you drive, but there are small differences in the actual process.

DIY or Drop Off?

Replacing your car battery is not an intensive process. While there are a few key steps to follow, most can do this on their own without an issue.

That said, it’s not ideal for those prone to anxiety or those who want to take as few chances with their vehicle as possible. Paying for someone else to do the work solves these issues, even if it multiplies your initial cost.

Paying for a professional eye is a good idea if you have other electrical issues with your vehicle that don’t line up with a dead auxiliary battery. They can run more in-depth diagnostics to ensure all your issues are resolved.

If budget is a major deciding factor, consider that auxiliary battery replacement runs anywhere from $250 to $450 at the dealership or your local mechanic. A DIY replacement only runs the cost of the battery, which ranges from $30 to $95.

Determining Location

Determining Location

If you plan on replacing the battery yourself, figure out where the small auxiliary battery is stored in your specific Mercedes model. This is usually one of five areas:

  • The driver’s side of the dashboard (E-class, C-class, S-class)
  • Inside the engine bay below the air intake of your HVAC
  • Under your front passenger seat (Most SUVs–GL, R, ML)
  • In the trunk near the fuse box
  • Behind the driver’s seat back panel (SLK convertible)

Find the location either by searching online or looking it up in your vehicle’s manual. Once you have the location you need, gather the tools specific to your situation.

Tools Needed

Regardless of where the battery is, you need to find a suitable replacement (make sure the part number matches yours exactly), a Torx socket set, and a trim removal tool.

All locations make use of a 10mm wrench, but you may need a 13mm wrench for a passenger side replacement. Double check these specifications for your specific model and year.

Replacing Your Mercedes-Benz Auxiliary Battery

  1. First, disconnect the negative cable from your main battery (either in your engine bay or trunk). This protects you from harmful discharge as you work.
  2. Locate your battery, removing any trim pieces or Torx screws in your way. For auxiliary batteries in the trunk, you may need to peel back some carpet to expose the right area.
  3. Remove any bolts that hold your battery bracket in place, then disconnect the cables from the battery (negative terminal first).
  4. Remove the battery completely, then reverse the process to install your new auxiliary battery.

Some areas, such as the dash, leave you very little room to work. Check online to see if there are other points of access that may provide you the room that you need.

Once you secure everything, the error code should clear.

This video won’t apply to every Mercedes-Benz vehicle, but it shows you the general process for safe removal and replacement.

If the Error Persists After a New Auxiliary Battery

There’s always the chance that the fault code doesn’t go away, and you’re stuck scratching your head and trying to figure out why the issue persists.

In some cases, the new battery discharged before you installed it. Hook it up to a voltmeter to make sure voltage is above 12 volts. If it’s less than this, give it some time to recharge and check again.

If the small auxiliary battery is fully charged and still posting a malfunction, look into other related issues.

Other Related Issues and Troubleshooting

Other parts and problems that might cause an auxiliary battery malfunction in Mercedes-Benz vehicles include:

  • Battery control module – usually more widespread performance issues
  • Auxiliary battery relay – battery still good but not charging
  • CAN communication issues (damaged wires or connectors)
  • Polyswitch fuse – F30; inspect fuse and replace if needed
  • Cut-off relay – interferes with proper charging
  • Voltage converter – manages output of regulator for full charge

Most Mercedes-Benz vehicles with the error code that have a functioning battery find that the voltage converter is their problem.

How to Replace the Voltage Converter (Capacitor) Failure [Most Common]

How to Replace the Voltage Converter (Capacitor) Failure [Most Common]

The voltage capacitor issue is common in most Mercedes models complaining of an auxiliary battery function. Current C-Class, G-Class, and GLC-Class models are at the top of the list, but all may have this issue eventually.

To replace the voltage converter, make sure you have the right part for your vehicle model and year and then follow the steps to locate your auxiliary battery. The voltage converter is usually only held on by a few small tabs, and you then reverse the process to install.

Make sure all your connections hold tight, then replace the panel covering your battery compartment and start your car again. This should resolve your issue.

Issue Persists After Battery Replacement and Troubleshooting

When you’ve run out of roads to explore and replace all the connected components, it’s better to hand the vehicle over to a professional for in-depth assessment. This may cost more, but it keeps you from running in circles.

You can drive your vehicle with the auxiliary battery malfunction, but this assumes that the auxiliary battery is the issue. Without proper diagnosis, you can’t understand the scope of the issue in your car’s electrical system or what safety features you’re missing.

It’s important to correct the issue as soon as possible to avoid a worst case scenario.


To fix the Mercedes auxiliary battery malfunction, start by replacing the auxiliary battery. This solves the issue in almost every scenario, is easy to do on your own, and is something you should do every few years anyway.

Remember to only perform work on your vehicle that you are comfortable with and implement all necessary safety measures. Hand the work off to a professional if it proves beyond the scope of your abilities or if you need a more timely repair.

Do you have a Mercedes auxiliary battery malfunction that you need to deal with? Comment any questions or concerns you still have.

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