2017 Acadia SLT - P0420 Code & Check Engine Light

ClearTheCode guidance (Gmc · Diagnostics & codes)

Summary: For 2017 Acadia SLT - P0420 Code & Check Engine Light, 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: Hey everyone, my 2017 GMC Acadia SLT with about 90k miles just threw a Check Engine Light. I scanned it with my OBD-II reader and got a P0420 code. It says 'Catalyst System Efficiency Below Threshold (Bank 1)'.

I haven't noticed any major performance issues, maybe a slight dip in MPG, but nothing drastic. What should I be looking at first? Could it be just an O2 sensor or is the catalytic convert…

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