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2018 Outlander GT: P0420 Code - Catalyst System Efficiency Below Threshold

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Mike T. · rep 0 · May 25, 4:05 AM · 1 replies · solved · Question

Vehicle: 2018 Mitsubishi Outlander GT

Hey everyone,

My 2018 Mitsubishi Outlander GT, with about 75,000 miles on it, just threw a Check Engine Light. I used my basic OBD-II scanner and it pulled a P0420 code: "Catalyst System Efficiency Below Threshold (Bank 1)".

The car seems to be running fine, no noticeable loss of power or strange smells. I've heard this could be a bad catalytic converter, but also maybe just an O2 sensor or an exhaust leak. I'm hoping it's not the cat, those are expensive!

Has anyone else experienced this with their Outlander? What was the fix? Any diagnostic steps I can take myself before heading to a shop? Thanks for any advice!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:05 AM

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ClearTheCode admin answer:

The P0420 code on your 2018 Mitsubishi Outlander GT indicates that the catalytic converter's efficiency is below the required threshold. While a failing catalytic converter is a possibility, it's crucial to diagnose thoroughly before replacement.

Here are the common causes and diagnostic steps:

  1. Exhaust Leaks: Even small leaks before or near the catalytic converter can introduce outside air, causing the O2 sensors to read inaccurately and trigger a P0420. Inspect the exhaust system, especially gaskets and welds, for any signs of leaks.
  2. Faulty Oxygen Sensors: While the P0420 specifically points to catalyst efficiency, a failing upstream or downstream O2 sensor can provide incorrect readings, leading the ECU to falsely believe the catalyst isn't working. A scan tool with live data can show O2 sensor waveforms; the downstream sensor should show a relatively steady voltage compared to the fluctuating upstream sensor if the cat is working correctly.
  3. Engine Misfires or Rich/Lean Conditions: Persistent engine misfires (P030x codes) or an engine running consistently too rich (P0172) or too lean (P0171) can damage the catalytic converter over time by overheating it or coating it with unburnt fuel/oil. Ensure there are no other active engine codes.
  4. Catalytic Converter Failure: If all other possibilities are ruled out, the catalytic converter itself may have degraded internally. This can happen due to age, contamination, or exposure to excessive heat.

When to see a shop:

If you've checked for exhaust leaks and don't have access to an advanced scan tool for live O2 sensor data, it's best to take your Outlander to a qualified mechanic. They can perform a more in-depth diagnosis, including:

  • Exhaust backpressure test to check for a clogged catalytic converter.
  • Detailed O2 sensor analysis using professional diagnostic equipment.
  • Visual inspection of the converter's internal structure if accessible.

Addressing the root cause is essential to prevent the code from returning and to ensure proper engine function and emissions control.

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