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2018 C300 Auxiliary Battery Malfunction Message

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Ashley M. · rep 0 · May 22, 4:15 AM · 1 replies · solved · Question

My 2018 Mercedes-Benz C300 (W205) recently started showing an "Auxiliary Battery Malfunction" message on the dashboard. It pops up intermittently, especially after starting the car. I've also noticed that the ECO start/stop function, which usually works flawlessly, is now completely inactive.

I'm wondering if this is a common issue for this model year and what the typical fix involves. Is it just a simple battery replacement, or could it be something more complex with the charging system?

The main battery seems fine, no issues with starting the car otherwise. Just this persistent message and the disabled start/stop. Any advice on troubleshooting or what to expect from the dealer?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:15 AM

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ClearTheCode guidance (Mercedes-Benz · Electrical & electronics)

Summary: For 2018 C300 Auxiliary Battery Malfunction Message, work through these steps on your 2016+ vehicle:

  1. Confirm the symptom — note when it happens (cold start, highway, after fuel fill, etc.).
  2. Scan codes — save all pending/stored DTCs; don't clear until you've recorded them.
  3. Check TSBs & recalls — look up your VIN for open campaigns and known fixes.
  4. Inspect the obvious — related fuses, connectors, fluid level, and recent repairs.
  5. Test the common fix — follow model-specific service procedures; one change at a time.
  6. Escalate if needed — burning smell, no brakes, overheating, or stalling in traffic → shop visit.

Owner reported: My 2018 Mercedes-Benz C300 (W205) recently started showing an "Auxiliary Battery Malfunction" message on the dashboard. It pops up intermittently, especially after starting the car. I've also noticed that the ECO start/stop function, which usually works flawlessly, is now completely inactive.

I'm wondering if this is a common issue for this model year and what the typical fix involves. Is it just…

This is educational guidance, not a substitute for hands-on diagnosis.

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