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2018 Outlander Sport - P0420 Code and Rough Idle

0

Diana O. · rep 0 · May 22, 5:28 AM · 1 replies · solved · Question

Hey everyone,

My 2018 Mitsubishi Outlander Sport, 2.0L FWD, just threw a Check Engine Light. I used my basic OBD-II scanner and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

Besides the light, I've noticed the car has been idling a bit rough lately, especially when cold. It feels like it's struggling a little, and the fuel economy seems to have dipped slightly. I'm wondering if these issues are related or if I'm looking at two separate problems.

What could be causing this P0420 code on a relatively new car? Is it definitely the catalytic converter, or could it be something else like an O2 sensor? Any advice on what I should check first before taking it to a mechanic?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 5:28 AM

0

ClearTheCode guidance (Mitsubishi · Diagnostics & codes)

Summary: For 2018 Outlander Sport - P0420 Code and Rough Idle, 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 Mitsubishi Outlander Sport, 2.0L FWD, just threw a Check Engine Light. I used my basic OBD-II scanner and it's showing a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'.

Besides the light, I've noticed the car has been idling a bit rough lately, especially when cold. It feels like it's struggling a little, and the fuel economy seems to have dipped slightly…

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

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