Mercedes Auxiliary Battery Malfunction Reset: 6 Steps

When your Mercedes displays an “Auxiliary Battery Malfunction” warning, it usually means there’s an issue with the small secondary battery that powers specific systems such as the start/stop function, COMAND, or emergency call features. This malfunction doesn't typically affect driving performance but should be resolved promptly.

Here’s a detailed guide on how to reset the auxiliary battery malfunction:

1. Diagnose the Problem
Before resetting, confirm that the auxiliary battery is the actual issue. Use an OBD-II scanner to read any error codes related to the battery or electrical system. Some common codes related to auxiliary battery faults include B15DC00 or B142E.

2. Locate the Auxiliary Battery
The position of the auxiliary battery depends on the Mercedes model. Common locations include:

• Behind the dashboard (e.g., E-Class W212)

• Inside the trunk near the main battery (e.g., S-Class W221)

• Under the passenger footwell
  Refer to your owner’s manual or a repair guide for the exact location.

3. Inspect and Replace if Necessary
Check the voltage of the auxiliary battery using a multimeter. If the reading is below 12 volts, it's likely weak or dead. In this case, replace it with an OEM or compatible aftermarket unit. Ensure the replacement battery matches the required specifications.

4. Clear the Fault Code / Reset the Warning
After replacing or recharging the auxiliary battery, the error message may remain. To reset it:

• Connect an OBD-II scanner that supports Mercedes-Benz systems.

• Navigate to the Body Control Module or Energy Management System.

• Select the auxiliary battery malfunction code and clear it.

• If no scanner is available, you can try disconnecting the main battery:

  1. Turn off the ignition.

  2. Disconnect the negative terminal of the main battery for at least 10–15 minutes.

  3. Reconnect the terminal and start the car.

5. Perform a Test Drive
Drive the vehicle for 10–15 minutes to confirm that the warning message no longer appears. If it returns, there may be a wiring issue, sensor fault, or the battery control module may require inspection.

6. Optional Software Reset (For Some Models)
In some cases, especially on newer models, resetting may require a Mercedes STAR diagnostic system or a qualified technician at a dealership to perform the reset properly.

Conclusion
Resetting the auxiliary battery malfunction is straightforward once the faulty battery is identified and replaced. However, persistent warnings may indicate deeper electrical issues that require professional diagnostics. Always ensure safety when working with electrical systems and disconnect the battery properly to avoid shorts or damage.