2018 Outlander P0420 Code - Catalyst Efficiency Below Threshold

ClearTheCode guidance (Mitsubishi · Diagnostics & codes)

Summary: For 2018 Outlander P0420 Code - Catalyst Efficiency Below Threshold, 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 Mitsubishi Outlander SEL just threw a Check Engine Light. I scanned it and got a P0420 code, which I know means catalyst system efficiency is below threshold.

The car seems to be running fine, no noticeable loss of power or strange noises. I'm wondering if this is something I can troubleshoot myself or if it's definitely a trip to the dealer.

Could it be an O2 sensor, or is it more likel…

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