2018 Civic Si Check Engine Light - P0420 Code

ClearTheCode guidance (Honda · Diagnostics & codes)

Summary: For 2018 Civic Si 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 Honda Civic Si just threw a Check Engine Light (CEL) yesterday. I scanned it with my OBD-II reader and got a P0420 code, which points to "Catalyst System Efficiency Below Threshold (Bank 1)". The car seems to be running fine, no noticeable loss of power or strange noises.

I'm wondering if this is something I can tackle myself or if it's a definite trip to the dealer. I've heard sometimes …

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