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2018 RDX Check Engine Light - P0420 Code

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Mark S. · rep 0 · May 22, 4:56 AM · 1 replies · solved · Question

Hey everyone, I'm hoping someone can help me out. My 2018 Acura RDX, Advance trim, just threw a Check Engine Light. I hooked up my OBD-II scanner and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be running fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure.

What are the common causes for this code on an RDX? Is it definitely the catalytic converter, or could it be something simpler like an O2 sensor? Any advice on troubleshooting before I take it to the dealer would be great.

✓ 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 RDX Check Engine Light - P0420 Code, 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, I'm hoping someone can help me out. My 2018 Acura RDX, Advance trim, just threw a Check Engine Light. I hooked up my OBD-II scanner and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

The car seems to be running fine, no noticeable loss of power or strange noises. Fuel economy might be slightly worse, but it's hard to tell for sure.

What are the co…

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

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