Auxiliary Battery Malfunction Mercedes-Benz Explained

Fix Mercedes-Benz Auxiliary Battery Malfunction

A Mercedes-Benz auxiliary battery malfunction refers to an issue with the secondary battery system in the vehicle. This battery is responsible for powering various electrical systems in the vehicle, such as the radio, navigation system, and some comfort features when the main battery is not providing power.

The auxiliary battery malfunctions may cause various electrical errors in the vehicle. It could lead to a dead main battery if not addressed. It’s important to have the vehicle inspected by a certified Mercedes-Benz technician to diagnose and repair the issue. In technical terms, the auxiliary battery malfunction can be caused by a variety of factors, such as:

  1. Aging or failing auxiliary battery: Over time, the auxiliary battery may lose its ability to hold a charge or may not be able to provide sufficient power to the electrical systems.
  2. Faulty battery management system (BMS): The BMS is responsible for monitoring and controlling the charging and discharging of the auxiliary battery. If the BMS is malfunctioning, it may not be able to charge or maintain the auxiliary battery properly.
  3. Wiring issues: Damaged or corroded wiring in the auxiliary battery circuit can prevent the battery from receiving power or properly communicating with the BMS.
  4. Faulty charging system: If the alternator or voltage regulator is not functioning properly, it may not be able to charge the auxiliary battery.

Auxiliary Battery Malfunction is a common error message when the Mercedes-Benz auxiliary battery fails. Mercedes-Benz uses an aux battery to operate the transmission and critical accessories in critical situations.

The most common issue that triggers this error on Mercedes-Benz vehicles is a dead auxiliary battery (PN: 000000004039).

In most cases, to fix Mercedes-Benz’s auxiliary battery malfunctions, you must replace the auxiliary battery. This repair takes about thirty minutes. The auxiliary battery location varies between models.

Mercedes auxiliary battery malfunction problem location

The auxiliary battery is typically located in one of these locations:

  • the left side of the dashboard (inside)
  • in the engine bay
  • under the front passenger seat.

In some models, the auxiliary battery may be in the engine bay (W211), under the passenger seat (most SUVs such as GL, R, ML), trunk, and behind the driver’s seat (SLK convertible); we will go into more detail later.

This guide will explain everything you need to know about the auxiliary and replace it yourself for under $100.

Newer Mercedes-Benz cars and SUVs have two batteries:

  • Main battery (G1) – System battery, starter battery, or G1. Its primary function is to start the engine and be the primary power source for all electrical systems.
  • Auxiliary battery (G1/7). – Also known as the secondary battery, backup battery, aux battery, or G1/7. Usually an AGM battery. Typically specs:  12V, 1.2Ah, 170 A.  Supplies power if the main battery (G1) voltage is low. Part number: 000000004039


auxilary battery

When the main battery dies, the car won’t start. The car starts when the auxiliary battery dies, but an auxiliary battery malfunction appears on the dashboard.

In most cases, the car runs and drives fine even with a bad auxiliary battery.

mercedes auxilary battery malfunction

Reading the fault codes with a Mercedes-Benz All System Scanner brings up the following fault code:

  • 9062 Component G1/7 (auxiliary battery) is defective or not installed. Status: Current and stored.

Some models, such as the early E-Class, and the auxiliary battery warning, show the battery symbol on the dashboard.

On newer models, when the auxiliary battery dies, you get a battery symbol and error message on the dashboard, such as:

  • Auxiliary Battery Malfunction
  • Stop Vehicle Leave Engine Running – can also be caused by the main battery.
  • Stop Vehicle Shift to P Leave Engine Running – can be caused by low voltage for the main battery.
  • Battery Symbol on Dashboard – can also be caused by a faulty alternator or voltage regulator.

Why does Auxilary Battery Malfunction come up?

Mercedes auxiliary battery

When the auxiliary battery fails to hold a charge to recharge, you will get an error message such as “auxiliary battery malfunction” or, in 2015 and newer models, “stop vehicle shift to P leave engine running” warning message. These two error messages are common for Mercedes-Benz cars, and other than the error message, the vehicle usually runs and drives fine.

If the car runs and drives fine, the problem is an auxiliary battery, but you keep seeing the auxiliary battery malfunction error on the dashboard.

We have seen Mercedes-Benz try to read the codes with a generic OBD-II scanner, but no fault codes come up. Using a generic scanner, you will not find fault codes that indicate a bad auxiliary battery since generic scanners cannot read Mercedes-Benz-specific fault codes.

You will know that the secondary battery is at fault because the car will still turn on, and some functions will work. The auxiliary battery does not affect the car’s mechanical functions.

What is the auxiliary battery used for?

function of mercedes auxiliary battery

One of the most important functions of the auxiliary battery is to operate the gear shift selector when the main car battery dies.

Mercedes-Benz has what is known as a drive-by-wire. This means your gear-select shifter is electronically controlled. If the main battery dies, you can no longer control the shifter and place the vehicle in Park. Since there is an auxiliary battery, you can still shift to Park even if the main battery dies. This is a safety feature.

The reason why the car runs and drives fine is that the auxiliary battery is hardly ever used. The early models, such as E-Class W211, were used to run electronic consumers, but that’s no longer the case. Depending on the model, the auxiliary battery can operate additional functions and accessories such as sensors, COMAND, heated seats, radars, stop-start functions, etc.

Where is the auxiliary battery located?

Mercedes benz auxiliary battery location

The most common location for the auxiliary battery on a Mercedes-Benz is on the left side of the dashboard. Other locations are where you will find auxiliary batteries on a Mercedes-Benz.

  • Engine bay – Passenger side.
    • E-Class W211 was installed below the air intake of the HVAC.
  • Under Passenger Seat
    • ML, R, and GL have the aux battery under the front passenger seat.
  • Behind the driver’s seat
    • SLK, SL has the auxiliary battery behind the driver’s seat back panel.
  • Trunk – Near fuse box.

The main location of the auxiliary battery is on the left side of the dashboard.

I replaced the auxiliary battery, but the warning still comes up.

First, check the new auxiliary battery is charged. We had seen it happen before when the new auxiliary battery we installed was discharged. You can use a voltmeter to measure the voltage of the auxiliary battery. It should be above 12 volts.

Several issues can trigger the auxiliary battery warning message:

  • Battery Control Module (N82). The battery control module monitors voltages and calculates the condition of both batteries. N82 monitors the voltages of (G1) and (G1/7)
  • Auxiliary battery relay (K57/2) Learn more about the K57/2 relay replacement here. 
  • CAN communication issue – Typically due to damaged wires or connector
  • Polyswitch Fuse (F30)
  • Auxiliary Battery Relay (K57/2) – Used to charge the auxiliary battery if the engine runs and the voltage is low.
  • Cut-off Relay for Interruptible Loads (K75) Cut-off Relay for Interruptible Loads (K75)

Even though many of the components above can cause auxiliary battery malfunction, we have found that 9 out of 10 times, the problem is the auxiliary battery itself.

Frequently Asked Questions

How do you replace the auxiliary battery on a Mercedes Benz?

First, you must determine where the auxiliary battery is on your Mercedes-Benz. Next, follow one of these guides on how to replace the auxiliary battery on a Mercedes-Benz.

How to Replace Aux Battery on the Side of Dashboard, Driver’s Side. 
How to Replace Aux Battery Under Front Passenger Seat
How to Replace Auxilary Battery in the Engine Bay
How to Replace Auxilary Battery Behind Driver’s Seat

Can I use an aftermarket auxiliary battery?

Yes. Aftermarket batteries that have similar specs as the original battery can be used. We have used aftermarket auxiliary batteries many times without any issues.

Which Mercedes-Benz models have an auxiliary battery?

Most 2003 and newer Mercedes-Benz have a secondary battery, also known as an auxiliary battery.
For example:
C-Class (W205 S205 C205)
S-Class (W221 W222),
E-Class (W213 S213 C238 W212 S212 C207 A207 A0009829308 auxiliary battery is located at the driver’s side.
GLC-Class (X253, C253),
SLK R171 R172
GL, GLS Class X164, and newer
R-Class W251
ML-Class W164

Most models, including E Class W211 S211 W212 S212 C207 A207 ML W166 GL X166 C class W204 S204 S class W220 W221 A-class W176 W177 CLA W117 CLS W218 C257 Sprinter W906 Vito W639 W447 B Class W246 W247 G Class W463 G500 G55 G63 AMG cars, have the small auxiliary battery.

If your Mercedes-Benz is equipped with the 722.9 7-speed transmissions, it will also have an auxiliary battery.

What is the difference between Secondary and Auxiliary Batteries?

Secondary and auxiliary batteries are often used interchangeably. There is a difference between the two.

Typically a secondary battery may be used to power several electronic consumers. Simultaneously, an auxiliary battery is mainly used as a backup to operate the electronic shifter if the main battery fails. Whether your Mercedes-Benz has a secondary or aux battery depends on the car model and year.

What is Mercedes-Benz N000000004039 auxiliary battery?

N000000004039 A000000004039 12V 1.2Ah lead-acid battery is the second model of the auxiliary battery, also known as the backup battery. It is used on many models, including Mercedes SUVs such as GL X164 GL450 GL550 GL350 GL320 S-class W220 W221 S320 CL-class  W216 CL550 CL600 CL63, R-class W251 R350, ML-class W164 ML250 ML300 ML350, etc.

What is the A0009829308 auxiliary battery?

One more type of battery for Mercedes cars is A0009829308, which is very similar to the previous model described above. Still, it has a slightly different shapes and locations for positive and negative terminals.

Can you recharge an auxiliary battery?

Mercedes-Benz auxiliary battery is a rechargeable 12V 1.2Ah battery. The alternator charges the auxiliary battery when the engine is running.
If the auxiliary battery voltage drops significantly, the alternator will stop charging it, and you will get an aux battery warning on the dashboard.
If the battery is still good, your only option is to remove the car’s battery and charge it with a 12 Volt Smart Charger, then reinstall.
If the auxiliary battery doesn’t hold a charge, it must be replaced.

How much does it Cost to Replace a Mercedes Auxiliary Battery?

The average cost to replace a Mercedes-Benz auxiliary battery at the dealer or auto mechanic is between $250 and $450.

The average cost to replace a Mercedes-Benz battery yourself is between $30 and $95.

Does an auxiliary battery need to be programmed?

No, the new auxiliary battery that you install does not need to be programmed. It will work without programming, and the Auxilary Battery Waring on the instruction cluster resets independently.

Can I drive the car with the auxiliary warning on?

When it comes to a Mercedes auxiliary battery malfunction, a certified repair shop is recommended to have it replaced as soon as possible. However, if the warning lights have come on, it is generally considered safe to drive for a short distance, such as up to 100 miles, before getting it repaired. It’s important to remember that driving with a malfunctioning auxiliary battery may cause various electrical systems in the vehicle to malfunction or not function at all.

Which Mercedes-Benz has an auxiliary battery?

Almost all 2005 and newer Mercedes-Benz vehicles have an auxiliary or secondary battery. Here are some examples:
C Cass W205 Saloon Sedan
C Class S205 Estate Touring
C Class A205 Cabriolet Convertible
C Class C205 Coupe
C Class 2014 2015 2016 2017 2018
C Class C180 C200 C220 BlueTec C250 C300 C350 c350e Hybrid C400 C450 C43 C450 C63 AMG
CLC Class W257 2017 2018 2019
E Class W213 Sedan
E Class S213 Estate
E Class C238 Coupe
E Class A238 Convertible
E Class 2016 2017 2018 2019
E Class W212 S212 A207 C207
E Class E200 E250 E300 E350e Hybrid E400 E450 E200d E220d E300d E350d E400d 4MATIC E43 E63 AMG BlueTEC
ML-class W164 ML250 ML300 ML350
GLE W166
R Class W251 R350 R500 R320 R63 AMG
GL X164 GL450 GL550 GL350 GL320
GLA-class X156 GLA180 GLA200 GLA250 GLA45 AMG GLA220 CDI
GLC X253 C253 W253 Hatchback Coupe 2015 2016 2017 2018 2019
GLC GLC300 GLC350e GLC220d GLC 220d GLC250d GLC350d GLC300 GLC43 AMG
S Class W220 W221 S320
CL-class  W216 CL550 CL600 CL63
S Class W222 Sedan 2013 2014 2015 2016 2017 2018
S Class C217 Coupe 2013 2014 2015 2016 2017 2018
S Class A217 Convertible Cabriolet 2013 2014 2015 2016 2017 2018
S Class S300 BlueTec S320 S350 S400 Hybrid S500 S550 S65 S63 AMG 4Matic

What is the part number of the Mercedes-Benz auxiliary battery?

The most common part numbers for a Mercedes-Benz auxiliary battery are:
A000 982 70 08


  • Rushit Hila • ASE Certified

    Rushit Hila, an ASE-certified engineer (G1 Automotive Maintenance and Repair), brings over two decades of hands-on experience in the automotive world to his writing. With a strong educational background, including a Master of Science in Engineering and a Bachelor of Science in Engineering, he has honed his skills and expertise through years of practical work. As a respected authority in the field, Mr. Hila is dedicated to offering insightful and valuable content that resonates with both vehicle owners and mechanics.

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  1. I own a 2016/17 GLA 180 (manual transmission) purchased from the sole agent. During my last visit to the agent for servicing, they informed me that the auxiliary battery was low and that a warning would appear after some time. I noticed that the “Auto Engine Stop” function was not activating as often as before, since the main function of the Aux. battery in my model is to run the feature comforts while the engine is off. I easily located the battery (at the front passenger footwell) and recharged it using a smart charger. Now, the Auto Engine Stop function activates more often, indicating that the Aux. battery has been given a new life. You can try this too! Also, in my model (and similar ones), the frequency of the Auto Stop function activation can serve as a tell-tale sign of the Aux. battery’s health.

  2. Peter Wischnowsky says:

    I have a right-hand drive 2009 cl coupe and we get the message abs system, not functioning. The car works fine but thought we should replace the auxiliary battery. We have ripped the car apart and cannot locate it! we have tried under the dash on both sides, the boot on all sides, the passenger seat back seat, you name it.
    It is a Japanese model exported to New Zealand, and has done 70,000 kms. All batteries have been tested and are OK, but we can’t find the auxiliary battery Location. I have purchased 000000004039 replacement from the USA a genuine Mercedes part.