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2018 RX 350 Check Engine Light - P0420 Code

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Melissa G. · rep 0 · May 22, 4:39 AM · 1 replies · solved · Question

Hey everyone, I'm hoping someone can help me out. My 2018 Lexus RX 350 F Sport just threw a Check Engine Light this morning. 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 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've heard this can be anything from an O2 sensor to a catalytic converter.

What should be my first steps to diagnose this? Is there anything I can check myself before taking it to a shop? Any common issues with this code on the RX 350?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:39 AM

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ClearTheCode guidance (Lexus · Diagnostics & codes)

Summary: For 2018 RX 350 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 Lexus RX 350 F Sport just threw a Check Engine Light this morning. 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 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've he…

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

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