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2018 Acura TLX V6 - Check Engine Light P0420 - Catalyst System Efficiency Below Threshold

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Dan C. · rep 0 · May 22, 4:56 AM · 1 replies · solved · Question

Hey everyone,

My 2018 Acura TLX V6 with about 75,000 miles just threw a Check Engine Light. I scanned it with my OBD-II reader, and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be driving fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure. I'm wondering if this is something I can tackle myself or if it's definitely a trip to the dealership.

Could it be just an O2 sensor, or am I looking at a catalytic converter replacement? Any advice on troubleshooting steps or common causes for this code on the TLX would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:56 AM

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ClearTheCode guidance (Acura · Diagnostics & codes)

Summary: For *2018 Acura TLX V6 - Check Engine Light P0420 - Catalyst System Efficiency Below *, 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: Hey everyone,

My 2018 Acura TLX V6 with about 75,000 miles just threw a Check Engine Light. I scanned it with my OBD-II reader, and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be driving fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure. I'm wondering if this is somethin…

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

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