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2020 Outlander Sport ES - Check Engine Light with P0420, Rough Idle

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Mike T. · rep 0 · May 24, 3:09 PM · 1 replies · solved · Question

Vehicle: 2020 Mitsubishi Outlander Sport ES

Hey everyone,

I'm hoping someone can offer some advice. My 2020 Mitsubishi Outlander Sport ES, with about 65,000 miles, recently threw a check engine light. I've also noticed a slight rough idle, especially when I'm stopped at a red light or in traffic. It's not severe, but definitely noticeable.

I hooked up my OBD-II scanner and pulled a P0420 code: 'Catalyst System Efficiency Below Threshold (Bank 1)'. I know this usually points to the catalytic converter or O2 sensors, but I'm trying to figure out the most common culprits for this specific model and year before I start throwing parts at it.

Has anyone else experienced this P0420 code or rough idle with their 2016-2026 Outlander Sport? What did you find was the cause, and what steps did you take to fix it? Any tips on what to check first would be greatly appreciated. Thanks in advance!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 3:09 PM

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ClearTheCode admin answer:

It sounds like you're on the right track with diagnosing the P0420 code. This code specifically indicates that the catalytic converter on Bank 1 is not operating at peak efficiency. While it often points to the converter itself, other issues can trigger it. Here's a breakdown of common causes and diagnostic steps:

  1. Understanding P0420: The engine's computer monitors the oxygen sensors before and after the catalytic converter. If the post-catalyst sensor's readings start to mirror the pre-catalyst sensor's readings too closely, it suggests the converter isn't storing enough oxygen, indicating reduced efficiency.

  2. Common Causes:

    • Failing Catalytic Converter: This is the most direct cause. Over time, converters can degrade due to age, rich fuel mixtures, or misfires.
    • Oxygen Sensors (O2 Sensors): A faulty upstream (pre-cat) or downstream (post-cat) O2 sensor can send incorrect data to the ECU, falsely triggering the P0420 code. The downstream sensor is particularly critical for this code.
    • Exhaust Leaks: Leaks in the exhaust system before or near the O2 sensors can allow unmetered air in, skewing sensor readings.
    • Engine Misfires or Rich/Lean Conditions: Persistent misfires, faulty spark plugs, or issues with fuel injectors can lead to unburnt fuel entering the exhaust, overheating and damaging the catalytic converter over time.

  3. Diagnostic Steps You Can Take:

    • Inspect for Exhaust Leaks: Visually check the exhaust system for any cracks, holes, or loose connections, especially around the manifold and catalytic converter.
    • Monitor O2 Sensor Data: If your scanner allows, monitor live data for both upstream and downstream O2 sensors. The upstream sensor should fluctuate rapidly, while the downstream sensor should show a relatively steady, higher voltage. If the downstream sensor mirrors the upstream, it strongly suggests a failing converter.
    • Check for Misfires: Look for any pending or active misfire codes (P030x) that could be contributing to converter degradation.

  4. When to See a Shop:

    • If you're not comfortable performing these diagnostic steps or if the issue persists after basic checks, it's best to consult a professional mechanic. They can perform more advanced tests, such as an exhaust backpressure test, to definitively diagnose the catalytic converter's health. Ignoring a P0420 code can lead to further damage to the exhaust system and may cause you to fail emissions tests.
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