2018 Enclave P0420 Code - Catalyst System Efficiency Below Threshold

ClearTheCode guidance (Buick · Diagnostics & codes)

Summary: For 2018 Enclave P0420 Code - Catalyst System 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 Buick Enclave Premium, with about 85,000 miles, recently illuminated the check engine light. I used my OBD-II scanner and it pulled a P0420 code, indicating "Catalyst System Efficiency Below Threshold (Bank 1)".

I haven't noticed any significant performance issues, but I feel like my fuel economy might have dipped slightly over the last few weeks. There's no strange exhaust smell or loud …

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