Mercedes-Benz Auxiliary Battery Malfunction & How to Replace It

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:

1. 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.
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 be unable to properly charge or maintain the auxiliary battery.
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 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

Symptoms

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.

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 – 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