2018 TLX A-Spec CEL P0420 - Catalyst Efficiency Issue

ClearTheCode guidance (Acura · Diagnostics & codes)

Summary: For 2018 TLX A-Spec CEL P0420 - Catalyst Efficiency Issue, work through these steps on your 2015+ 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 Acura TLX A-Spec, with about 75k miles, recently threw a Check Engine Light. I scanned it with my OBD-II tool, and it's showing P0420, which points to Catalyst System Efficiency Below Threshold (Bank 1).

I haven't noticed any significant performance issues or strange smells, but the light is persistent. I cleared it once, and it came back after about 50 miles of driving.

Could this be so…

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