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2018 Acadia SLT - P0420 Code and Reduced Power

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Robert C. · rep 0 · May 22, 3:39 AM · 1 replies · solved · Question

My 2018 GMC Acadia SLT, equipped with the 3.6L V6 engine, just threw a Check Engine Light. I scanned it with my personal OBD-II reader and got a P0420 code, which indicates "Catalyst System Efficiency Below Threshold (Bank 1)".

I've noticed the car feels a bit sluggish, especially on acceleration, and the fuel economy seems to have dipped slightly. I'm worried about potential damage to the catalytic converter or other components if I keep driving it like this.

Has anyone experienced this specific code on their Acadia? What were the common culprits and fixes you found? I'm trying to figure out if this is something I can tackle myself or if it's definitely a shop visit.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:39 AM

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ClearTheCode guidance (Gmc · Diagnostics & codes)

Summary: For 2018 Acadia SLT - P0420 Code and Reduced Power, 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 GMC Acadia SLT, equipped with the 3.6L V6 engine, just threw a Check Engine Light. I scanned it with my personal OBD-II reader and got a P0420 code, which indicates "Catalyst System Efficiency Below Threshold (Bank 1)".

I've noticed the car feels a bit sluggish, especially on acceleration, and the fuel economy seems to have dipped slightly. I'm worried about potential damage to the cataly…

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

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