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2018 C300 Rough Idle & Check Engine Light - M274 Engine Concerns?

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Angela R. · rep 0 · May 22, 4:22 AM · 1 replies · solved · Question

Hey everyone,

My 2018 Mercedes-Benz C300 (W205 chassis, M274 engine) has developed a really rough idle lately. It feels like the car is shaking more than usual, especially when stopped at a red light. The check engine light also came on a few days ago and hasn't gone off since.

I'm worried about potential engine issues, as I've heard some mixed things about the M274 engine's reliability. Is this a common problem with the C300s from this year range? What should I be looking for, or what are the typical culprits for a rough idle and CEL on these cars?

Any advice or shared experiences would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode guidance (Mercedes-Benz · Buying & ownership)

Summary: For 2018 C300 Rough Idle & Check Engine Light - M274 Engine Concerns?, 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: Hey everyone,

My 2018 Mercedes-Benz C300 (W205 chassis, M274 engine) has developed a really rough idle lately. It feels like the car is shaking more than usual, especially when stopped at a red light. The check engine light also came on a few days ago and hasn't gone off since.

I'm worried about potential engine issues, as I've heard some mixed things about the M274 engine's reliability. Is this…

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

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