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Mercedes-Benz C 300 Auxiliary Battery Malfunction - How To Troubleshoot and Fix

What Does The Auxiliary Battery In A Mercedes Do?

Your Mercedes-Benz C 300’s auxiliary battery is an integral part of your vehicle’s operation. Without it, your instrument cluster, dashboard error messages, and other electrical components such as GPS will not turn on, impacting the overall driving experience.

Keep reading to learn more about auxiliary batteries, auxiliary battery malfunction C 300, causes to malfunction, and fixing your C 300 auxiliary battery.


What Is An Auxiliary Battery?

Auxiliary batteries work with your main starter battery, and as a backup battery in the event the main battery becomes disabled. It prevents your vehicle’s electrical components from dropping when starting/stopping your engine and keeps your radio, GPS, and other in-vehicle electrical components turned on.

Any failure with your voltage converter will typically depict an error fault message on your instrument cluster. It may appear as a small battery symbol or show an onboard computer error fault message displaying an “auxiliary battery malfunction.” You will notice it immediately every time you start your vehicle until a proper diagnosis (and potential replacement) by yourself or a licensed technician can be performed.


Mercedes C 300 Auxiliary Battery Location.

Your Mercedes-Benz C 300 auxiliary battery is located under your hood, beneath your cabin air housing. It takes no more than a minute to access. Simply pop up your hood using your hood release lever and remove the cabin air filter housing to reveal the auxiliary battery.


How Does a Faulty Auxiliary Battery Affect My Vehicle’s Performance?

An auxiliary battery malfunction can cause your vehicle’s electrical systems and operations to display errors. These include your entertainment system, GPS, PDC sensors, radio, and start/stop functionality. You will start to notice vehicle safety features like Blind Spot Assist, Lane Keeping Assist, and similar lighting notifications fail to activate. An AUX battery light may also activate in your instrument cluster.

Note, these are not critical components, so it won’t cause your vehicle not to start. However, they can affect your concentration on the road, especially if you are reliant on any of these helpful safety features like Lane Assist. Imagine having to drive without your radio on during a long road trip? Your fellow passengers may not be able to forget it for months afterward.


How to Replace Your Mercedes-Benz C 300 Auxiliary Battery.

We always recommend taking your vehicle in to a certified technician but fixing the C300 auxiliary battery can be very easy. This process should take no more than 20 minutes for even novice mechanics who have never been under the hood before.

In the event that you’re unable to use a certified technician or you want to do this yourself, here’s how to accurately troubleshoot:

  1. Activate your emergency brakes and pull the driver’s side hood release to lift up the hood.
  2. To the left of your engine bay, locate the auxiliary battery under your cabin air filter housing. Remove the cabin air filter’s bolt and then remove the cabin air filter housing to reveal the auxiliary battery.
  3. Use a 10mm socket wrench to remove the negative battery terminal, followed by the positive battery terminal. From there, remove the battery bracket using the same tool.
  4. Remove the old auxiliary battery and install the new battery by fastening the brackets in place and connecting the positive terminal, followed by the negative terminal. Pop back the cabin air filter housing over your auxiliary battery to complete the process.


With your new auxiliary battery, ensure it comes in the standard 12V configuration and specific to your Mercedes-Benz C 300. A licensed technician is more than capable of providing the correct recommendation. You can expect your auxiliary battery to deteriorate over time before failing completely. On average, auxiliary batteries have a service life of up to seven years, more or less depending on usage and driving conditions.

This process should take no more than 20 minutes from start to finish. Someone with limited mechanical skills could easily take this on.

Your Mercedes-Benz C 300’s auxiliary battery works to keep your most critical driving visuals such as Lane Assist, GPS, radio, and other electrical components turned on. Again, though it may not directly affect driving performance, you also have the option to consult a licensed technician to diagnose and replace if needed.