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2018 C300 Auxiliary Battery Malfunction & Start/Stop Not Working

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Rachel A. · rep 0 · May 22, 4:14 AM · 1 replies · solved · Question

My 2018 Mercedes-Benz C300 (W205) has been showing an "Auxiliary Battery Malfunction" message on the dash for a few weeks now. Also, the ECO start/stop function has completely stopped working.

I've noticed the car sometimes struggles a bit to start, especially on colder mornings, but it always eventually turns over. I thought it might just be the main battery, but the message specifically mentions the auxiliary one.

Is this a common issue? What's involved in replacing the auxiliary battery, and could it be something more serious than just a battery swap?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:14 AM

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ClearTheCode guidance (Mercedes-Benz · Electrical & electronics)

Summary: For 2018 C300 Auxiliary Battery Malfunction & Start/Stop Not Working, 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) has been showing an "Auxiliary Battery Malfunction" message on the dash for a few weeks now. Also, the ECO start/stop function has completely stopped working.

I've noticed the car sometimes struggles a bit to start, especially on colder mornings, but it always eventually turns over. I thought it might just be the main battery, but the message specifically mention…

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

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