Fixing The "Auxiliary Battery Malfunction" On A Mercedes (2024)

If you are a driver of a Mercedes-Benz, you may have gotten a message on the dashboard saying, “Auxiliary Battery Malfunction.” But what does this warning light mean, and how can you fix it? It’s vital to know how to repair the Auxiliary Battery Malfunction in Mercedes models if you want to have a reliable drive.

In this guide, I discuss the auxiliary battery and talk about its location. I will also show you the differences between the main and auxiliary batteries, both of which are important, but for different reasons. Toward the end of this guide, I cover the top symptoms, causes, and fixes of this electrical system fault.

What Does The Auxiliary Battery Malfunction Message Mean?

The Auxiliary Battery Malfunction warning message indicates that something is wrong with the secondary battery system on your Mercedes-Benz vehicle. These batteries – sometimes known as a capacitor – are responsible for secondary functions, such as running electrical components.

When the efficiency and voltage drop too low for the auxiliary battery and the car’s electronic monitoring system detects that, it can result in this warning message on your dashboard.

What is the Auxiliary Battery?

Fixing The "Auxiliary Battery Malfunction" On A Mercedes (1)

Every passenger vehicle comes equipped with a 12-volt battery that is used to start its engine. The same is true with the Mercedes-Benz lineup of vehicles. However, the German automaker has also installed a second battery in vehicles since the early 2000s.

This second battery is known as the auxiliary battery. It’s a smaller version of the battery that starts the vehicle. In some cases, Mercedes refers to it as a capacitor because of its smaller size.

This auxiliary battery serves as a backup to the main car battery. Depending on which Mercedes model it’s installed in, it serves a different purpose. In some cases, the auxiliary battery or capacitor is used to power up the electrical accessories, such as the infotainment system or radio. It can also be used for the engine’s stop-start system operation.

As with the main car battery, this secondary battery gets charged any time that the engine runs. It also has a similar lifespan, ranging from four to six years. Once it can no longer hold a charge, you might see a warning light illuminate on the dashboard or infotainment system.

Where is the Mercedes Auxiliary Battery Located?

Mercedes-Benz puts the auxiliary battery in multiple locations, depending on which model you drive. If you are looking to replace the secondary battery yourself, you need to know where to look.

The common locations where to find the auxiliary battery on a Mercedes include:

  • In front of the windshield in the engine bay
  • Underneath the front passenger seat
  • Inside the dashboard on the driver’s side
  • Within the engine bay
  • In the trunk
  • Behind the driver’s seat

If you aren’t able to find the auxiliary battery in your Mercedes, you can find the location in your service manual. There are also diagrams available online if you need more support.

Main vs. Auxiliary Battery

The main battery in your Mercedes vehicle is used in every passenger vehicle. It’s also known as the starter battery, system battery or G1. It’s needed to start the motor, and it provides the primary source of power to the car’s electrical systems.

In comparison, the auxiliary battery, known as the G1/7, is the secondary battery. It can also be called the aux battery or backup battery. This battery is usually an AGM battery but could also be a capacitor, depending on how big it is. In the Mercedes, this battery isn’t used for starting the engine, but it does provide support for the electrical components.

Symptoms of Bad Auxiliary Battery Malfunction

If your auxiliary battery is bad, you might see the auxiliary battery malfunction warning light appear on the dashboard. This could be the first warning that the battery isn’t charging as it should. Still, the car will continue running and driving as normal.

With this warning message, your code scanner might be able to pick up the “9062 Component G1/7 (auxiliary battery) is defective or not installed” DTC. Aside from this, you might start to notice trouble with the electrical system features. Depending on what the secondary battery runs, the trouble could be with the infotainment system, radio, and more.

It’s simple to tell when the main car battery dies. When this happens, the motor no longer starts. On the other hand, when the auxiliary battery dies, the symptoms may not be as recognizable.

What Causes An Auxiliary Battery Malfunction Message?

The most common cause of an auxiliary battery error message is a bad auxiliary battery, a bad auxiliary battery relay, or a blown PolySwitch fuse. It can also be caused by a bad battery control module or a wiring problem.

Here is some more detailed information about the causes of an auxiliary battery error message:

1. Defective Auxiliary Battery

I’ve already discussed the main reason for the Auxiliary Battery Malfunction message. In most normal cases, the trouble will be the battery itself.

As the battery ages, it needs to be replaced, just as your car battery would. This message might give you enough warning to know when the battery is dying before it’s completely dead.

2. Malfunctioning Auxiliary Battery Relay

The battery relay has the important job of deciding when the auxiliary battery needs to be charged. If the voltage drops, the relay sends out the message, ensuring it gets recharged.

For this function to work, the engine must be running. Once the relay fails, a warning light can illuminate, and the battery will cease charging.

3. Bad PolySwitch Fuse

The PolySwitch fuse works like a normal fuse, in that it protects from surges and overcurrents. It keeps the flow of electricity within normal ranges.

The difference between the PolySwitch fuse and a typical automotive fuse is that this device will reset itself once a fault gets cleared. Still, if there’s trouble with the PolySwitch fuse, the Auxiliary Battery Malfunction light can come on.

4. Bad Battery Control Module

This module is responsible for measuring the voltage. It handles the monitoring of both the primary and secondary batteries.

If this module fails, you can see the Auxiliary Battery Malfunction warning light. It’s also possible to illuminate the primary battery warning light too.

5. CAN Communication Issue

The controller area network (CAN) bus is a networking device that is used in automotive applications. If there’s a problem with this system, it can cause the Auxiliary Battery Malfunction error message.

However, it’s not normally a problem with the CAN bus itself. In most cases, the trouble stems from a bad connector or damaged wiring.

How to Fix a Mercedes with Auxiliary Battery Malfunction?

To fix an auxiliary battery malfunction message on your Mercedes, you should start by scanning the error codes with a diagnostic scanner. Then you need to diagnose the auxiliary battery, relay, and fuse and replace it if it is broken.

Here is some more detailed information on how to fix a Mercedes with auxiliary battery problems:

1. Scan Trouble Codes

Any time you are working with a dashboard warning light, the first step would be to use your code scanner to see what the system is trying to tell you. If there is an abundance of codes set, you might want to reset them, go for a short drive and scan again.

In some cases, you may only get generic codes that make it difficult to determine what’s wrong. Other times, there could be multiple codes that paint a more complete picture of what’s going on.

2. Replace Old Auxiliary Battery

If the trouble codes lead you to believe that the problem is with the auxiliary battery, it’s best to replace it first. In most cases, this warning light comes on because the battery stops charging after being used for many years.

For this reason, any battery more than four years old should be replaced. You can perform this task yourself with some basic tools. The hardest part might be finding the location of the auxiliary battery.

RELATED: How to Check Car Battery Health at Home (8 Steps)

3. Diagnose Electrical System

Fixing The "Auxiliary Battery Malfunction" On A Mercedes (2)

If changing the auxiliary battery doesn’t resolve the issue, there’s something larger going on. You need to inspect the battery’s relay and the fuse. I’ve covered a few of the parts you need to troubleshoot to get to the bottom of what’s happening.

Use your Mercedes service manual or a repair manual to walk through the appropriate steps. Once you find the faulty part or connection, you can repair it.

4. Seek Help from a Professional

In some cases, the diagnosis might be over your head. Whether you don’t have the right code scanner, or you aren’t versed in working on your Mercedes, it might be best to visit a local repair shop.

When it comes to working with Mercedes vehicles, you want to visit a specialty shop. In many ways, these luxury automobiles are different from the competition, and it’s best to trust a professional.

Can I drive with an auxiliary battery malfunction?

Yes. You can drive with a bad auxiliary battery, but it is not recommended to do so. Your car will still run from the main battery, but you may notice some electrical problems due to the bad auxiliary battery, depending on the car model. Therefore, it is recommended to fix the problem as soon as possible.

What happens if the auxiliary battery dies?

If your auxiliary battery dies in your Mercedes, in most cases, you will only notice an auxiliary battery malfunction message on your dashboard. You may also notice the 9062 DTC code if you scan the vehicle with an OBD scanner. In rare cases, you may also notice other electronic problems in your car.

Does the alternator charge the auxiliary battery?

Yes. The alternator charges both the main and auxiliary batteries when the engine is running. There is a relay for the auxiliary battery that opens and closes the circuit when the auxiliary battery should be charged by the generator.

Do you have to use a Mercedes auxiliary battery?

No. You don’t need to buy a Mercedes auxiliary battery if you know what you’re doing. But when choosing a battery, it is very important that you choose a battery with the right specifications. Therefore, if you are not sure, it is better to choose a Mercedes auxiliary battery for your specific car model.


The auxiliary battery malfunction message is most often caused by a bad auxiliary battery or its relay or fuse. If you know it’s been a very long time since the auxiliary battery was replaced in your Mercedes, you may want to replace it even if it isn’t causing the problem right now.

After reading this blog post, you should now be aware of the auxiliary battery and why you should not ignore the problem. You should always make sure to check both your batteries regularly and replace them when necessary to avoid issues.

Learn more:

  • Are Mercedes Benz Good Cars? Are They Reliable?
  • How Long Does a Car Battery Last?
  • How Much Does It Cost to Replace a Car Battery?

As an automotive enthusiast and expert in the field, I can attest to the critical role that auxiliary batteries play in modern vehicles, particularly in high-end brands such as Mercedes-Benz. The information provided in the article aligns with my extensive knowledge of automotive electrical systems, and I can further elaborate on the concepts discussed.

1. Purpose of the Auxiliary Battery: The article accurately describes the auxiliary battery as a secondary power source responsible for supporting various electrical components in a Mercedes-Benz vehicle. It supplements the main battery by providing power for secondary functions such as the infotainment system, radio, and engine stop-start system operation.

2. Location of the Auxiliary Battery: Mercedes-Benz, known for its engineering sophistication, places the auxiliary battery in multiple locations depending on the model. The article correctly highlights common locations, including under the front passenger seat, in the engine bay, inside the dashboard, in the trunk, behind the driver's seat, and in front of the windshield. This variation underscores the brand's commitment to optimizing space and performance.

3. Distinction Between Main and Auxiliary Batteries: The differentiation between the main (starter) battery and the auxiliary battery is crucial, and the article appropriately delineates their roles. The main battery is essential for starting the engine and powering primary electrical systems, while the auxiliary battery supports secondary components.

4. Symptoms and Causes of Auxiliary Battery Malfunction: The symptoms of a bad auxiliary battery, as mentioned in the article, align with my expertise. The warning light on the dashboard, potential trouble with electrical features, and specific diagnostic trouble codes (DTC) like "9062 Component G1/7 (auxiliary battery) is defective or not installed" are indicative of an issue.

5. Causes of Auxiliary Battery Malfunction: The detailed breakdown of potential causes, including a defective auxiliary battery, malfunctioning relay, bad PolySwitch fuse, faulty battery control module, and CAN communication issues, reflects a deep understanding of the complex interplay within the vehicle's electrical system.

6. How to Fix Auxiliary Battery Issues: The article provides a comprehensive guide on fixing auxiliary battery malfunctions, emphasizing the importance of scanning trouble codes, replacing the auxiliary battery if necessary, diagnosing the electrical system, and seeking professional help if needed. This aligns with standard procedures in automotive diagnostics and repair.

7. Driving with a Bad Auxiliary Battery: The advice on driving with a bad auxiliary battery is pragmatic but cautious. While it is possible to drive with a malfunctioning auxiliary battery, the article rightly recommends addressing the issue promptly to avoid potential electrical problems.

8. Alternator Charging the Auxiliary Battery: The clarification that the alternator charges both the main and auxiliary batteries aligns with fundamental automotive principles. The mention of a relay controlling the charging circuit for the auxiliary battery adds technical precision to the explanation.

9. Choosing a Replacement Auxiliary Battery: The article's guidance on choosing a replacement battery emphasizes the importance of selecting one with the right specifications. While it acknowledges that a Mercedes auxiliary battery is not mandatory, it highlights the significance of compatibility for optimal performance.

In conclusion, the information provided in the article is accurate, well-detailed, and aligns seamlessly with my extensive knowledge of automotive systems. It serves as a valuable guide for Mercedes-Benz owners facing auxiliary battery malfunctions, demonstrating a deep understanding of the subject matter.

Fixing The "Auxiliary Battery Malfunction" On A Mercedes (2024)
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