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2018 C300 Auxiliary Battery Malfunction Message

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Mike T. · rep 0 · May 25, 3:07 AM · 1 replies · solved · Question

Vehicle: 2018 Mercedes-Benz C300 Sedan

Hey everyone,

I've been getting an 'Auxiliary Battery Malfunction' warning on my dashboard for the past few weeks. It pops up intermittently, usually when I first start the car or after a short drive. My 2018 Mercedes-Benz C300 Sedan has about 65,000 miles on it.

I've noticed that the ECO start/stop function no longer works, and sometimes the gear selector feels a bit sluggish when shifting into drive or reverse. Could this really just be a small auxiliary battery, or is it indicative of a larger electrical issue? I replaced the main battery about a year ago, so I don't think that's the culprit.

Any advice on what to check or if this is something I can tackle myself before heading to the dealership? Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 3:07 AM

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ClearTheCode admin answer:

The 'Auxiliary Battery Malfunction' message in your 2018 Mercedes-Benz C300 is a common issue and usually points directly to the small auxiliary battery itself. This battery is distinct from your main starting battery and primarily powers non-essential systems like the ECO start/stop function, gear selector module, and sometimes even the electronic parking brake during specific conditions.

Here's what's likely happening and what to do:

  1. Understanding the Auxiliary Battery: Over time, these batteries degrade and lose their capacity to hold a charge, especially after 4-6 years. When it fails, systems that rely on its stable voltage, like the ECO start/stop, will cease to function, and you might experience minor glitches with other electronics.

  2. Common Causes:

    • Age: The most frequent cause. Batteries have a finite lifespan.
    • Voltage Drop: Even if it's charging, it might not be holding sufficient voltage under load.
    • Internal Resistance: As the battery ages, internal resistance increases, reducing its ability to deliver current.

  3. Diagnosis and Action:

    • Check for OBD Codes: A diagnostic scan might reveal codes like B182F (Auxiliary Battery Voltage Fault) or similar, confirming the issue.
    • Voltage Test: While possible to test with a multimeter, the best way to confirm is a load test, which most auto parts stores can do for free.
    • Replacement: The auxiliary battery is typically located in the trunk or under the passenger footwell, depending on the specific C-Class variant. It's often a relatively straightforward DIY replacement, but ensure you use a genuine Mercedes-Benz part or an equivalent high-quality AGM battery of the correct specifications.

  4. When to See a Shop:

    • If replacing the auxiliary battery doesn't resolve the issue.
    • If you're uncomfortable working with vehicle electrical systems.
    • If a diagnostic scan reveals multiple, unrelated electrical codes, suggesting a more complex issue with the charging system or a control module. A professional can properly diagnose and ensure all systems are functioning correctly after replacement.
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