2018 C300 Check Engine Light - P0420 Code

ClearTheCode guidance (Mercedes-Benz · Diagnostics & codes)

Summary: For 2018 C300 Check Engine Light - P0420 Code, 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 C300, 4MATIC, just threw a check engine light. I scanned it with my OBD-II tool, and it's showing a P0420 code: "Catalyst System Efficiency Below Threshold (Bank 1)". The car seems to be driving fine, no noticeable loss of power or strange noises.

I'm a bit worried about what this could mean. Is it definitely the catalytic converter, or could it be something simpler like an O2 sensor? I'v…

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