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2018 C300 Check Engine Light - P0420 and P0301 Codes

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Sarah K. · rep 0 · May 22, 4:12 AM · 1 replies · solved · Question

Hey everyone, I'm hoping someone can help me out. My 2018 Mercedes-Benz C300 (W205 chassis) just threw a Check Engine Light this morning. I pulled out my OBD-II scanner and got two codes: P0420 (Catalyst System Efficiency Below Threshold Bank 1) and P0301 (Cylinder 1 Misfire Detected).

The car seems to be running okay, maybe a slight hesitation at idle, but nothing major. I'm worried about the P0420 code, as I know catalytic converters can be expensive. Could the misfire be causing the P0420, or are these separate issues?

I've checked the gas cap, and it seems fine. Any ideas on where to start troubleshooting? Should I try clearing the codes first?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:12 AM

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ClearTheCode guidance (Mercedes-Benz · Diagnostics & codes)

Summary: For 2018 C300 Check Engine Light - P0420 and P0301 Codes, 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, I'm hoping someone can help me out. My 2018 Mercedes-Benz C300 (W205 chassis) just threw a Check Engine Light this morning. I pulled out my OBD-II scanner and got two codes: P0420 (Catalyst System Efficiency Below Threshold Bank 1) and P0301 (Cylinder 1 Misfire Detected).

The car seems to be running okay, maybe a slight hesitation at idle, but nothing major. I'm worried about the P0…

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

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