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2017 C300 Auxiliary Battery Warning & ECO Start/Stop Failure

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Mark S. · rep 0 · May 25, 3:07 AM · 1 replies · solved · Question

Vehicle: 2017 Mercedes-Benz C300

Hey everyone,

I've been getting an 'Auxiliary Battery Malfunction' warning on my 2017 Mercedes-Benz C300 (W205 chassis) for the past few weeks. It pops up intermittently, usually when I first start the car. Along with this, my ECO start/stop function has completely stopped working. It used to work flawlessly, but now it never engages.

I've read online that this is a common issue and often points to the auxiliary battery itself or a voltage regulator. Has anyone experienced this specifically on a 2017 C300? I'm trying to figure out if this is something I can tackle myself or if it's definitely a dealer visit.

Any advice on diagnostics or what parts I might need would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

This is indeed a very common issue for Mercedes-Benz vehicles, especially in the 2015-2020 range, and your diagnosis is likely correct. The 'Auxiliary Battery Malfunction' warning and non-functional ECO start/stop are classic symptoms of a failing auxiliary battery or its control module/voltage converter.

  1. Causes:

    • Failing Auxiliary Battery: Over time, these smaller batteries, often located in the trunk or under the dashboard, lose their capacity. They are crucial for powering secondary systems like the ECO start/stop, gear selector, and other electronics during the brief engine shutdown.
    • Auxiliary Battery Voltage Converter/Module: In some Mercedes models, a dedicated voltage converter manages the auxiliary battery. If this module fails, it can prevent the auxiliary battery from charging or functioning correctly, triggering the warning.
    • Main Battery Issues (Less Likely but Possible): While less common for this specific warning, a weak main starter battery can sometimes indirectly affect the auxiliary system if the charging system isn't operating optimally.

  2. Diagnostics:

    • A basic OBD-II scanner might not show specific codes for the auxiliary battery, but a Mercedes-specific diagnostic tool (like XENTRY/DAS) would likely show codes related to the auxiliary battery circuit (e.g., B21DC00 - 'The voltage of the auxiliary battery is too low' or B21DD00 - 'The auxiliary battery has a malfunction').
    • Visually inspect the auxiliary battery (location varies by model, often in the trunk near the spare tire or under the passenger footwell) for any signs of corrosion or swelling.

  3. When to See a Shop:

    • DIY Potential: Replacing the auxiliary battery itself can be a DIY job for those comfortable with basic car maintenance. The battery is typically accessible, though some trim removal may be required. Ensure you use the correct type and size of battery.
    • Professional Help Recommended: If, after replacing the auxiliary battery, the warning persists, or if you suspect the voltage converter/module, it's best to consult a Mercedes-Benz specialist or dealership. These modules often require specific coding or adaptation after replacement, which only specialized diagnostic tools can perform. Ignoring this issue can lead to other electrical glitches or prevent critical safety systems from functioning correctly.
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