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P0420 Code on my 2017 Optima - Catalyst System Efficiency Below Threshold

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Emily N. · rep 0 · May 25, 2:06 AM · 1 replies · solved · Question

My check engine light came on last week in my 2017 Kia Optima LX with about 85,000 miles. I used my cheap OBD-II scanner and it pulled a P0420 code, which says "Catalyst System Efficiency Below Threshold (Bank 1)".

The car seems to be running fine, maybe a slight dip in MPG, but nothing major. I'm wondering if this is something I can tackle myself or if it's a guaranteed trip to the mechanic. What are the common culprits for this code on Kias?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:06 AM

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ClearTheCode guidance (Kia · Diagnostics & codes)

Summary: For P0420 Code on my 2017 Optima - Catalyst System Efficiency Below Threshold, 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 check engine light came on last week in my 2017 Kia Optima LX with about 85,000 miles. I used my cheap OBD-II scanner and it pulled a P0420 code, which says "Catalyst System Efficiency Below Threshold (Bank 1)".

The car seems to be running fine, maybe a slight dip in MPG, but nothing major. I'm wondering if this is something I can tackle myself or if it's a guaranteed trip to the mechanic. Wha…

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

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