A Mercedes-Benz auxiliary battery malfunction refers to an issue with the secondary battery system in the vehicle. This aux battery is responsible for powering various electrical systems in the vehicle but, most critically, allowing the electronically controlled transmission to shift in Park if the main battery power is cut.
The auxiliary battery malfunctions may cause various electrical errors in the vehicle. 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:
- Aging or failing auxiliary battery: Over time, the auxiliary battery may lose its ability to hold a charge or be unable to provide sufficient power to the electrical systems.
- 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 be unable to properly charge or maintain the auxiliary battery.
- Wiring issues: Damaged or corroded wiring in the auxiliary battery circuit can prevent the battery from receiving power or properly communicating with the BMS.
- Faulty charging system: If the alternator or voltage regulator is not functioning properly, it may be unable 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.
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, starter, 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
When the main 12-volt battery dies, the car won’t start, but the car will still start with a dead auxiliary battery. Even though the car starts when the auxiliary battery dies, an auxiliary battery malfunction appears on the dashboard. In most cases, the car runs and drives fine, even with a bad auxiliary battery.
- 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 – The main battery can also cause this.
- Stop Vehicle Shift to P Leave Engine Running – This 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?
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 see 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. You will not find fault codes that indicate a bad auxiliary battery using a generic scanner since generic scanners cannot read Mercedes-Benz-specific fault codes.
One good example of a multi-system scanner is the YOUCANIC Full Systems Scanner. This scanner can read and clear fault codes from all the systems in your Mercedes-Benz. It can also perform bi-directional tests, maintenance and repair resets, etc.
You will know 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 starting functions.
What is the auxiliary battery used for?
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 true. 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?
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 auxiliary battery’s main location is on the dashboard’s left side.
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 specs similar to 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.
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
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 the 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 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.
It must be replaced if the auxiliary battery doesn’t hold a charge.
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 you install does not need to be programmed. It will work without programming, and the auxiliary 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 replace it as soon as possible. However, if the warning lights have come on, driving for a short distance, such as up to 100 miles, is generally considered safe 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
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