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2020 Outlander Sport P0420 Code - Catalyst System Efficiency Below Threshold

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Jason D. · rep 0 · May 22, 5:31 AM · 1 replies · solved · Question

Vehicle: 2020 Mitsubishi Outlander Sport ES

Hey everyone,

My Check Engine Light just came on in my 2020 Mitsubishi Outlander Sport ES. I went to AutoZone and had them pull the code, and it's showing P0420: Catalyst System Efficiency Below Threshold (Bank 1).

The car seems to be running okay, maybe a slight dip in MPG, but nothing major. I'm wondering if this is something I can tackle myself or if it's definitely a trip to the mechanic.

What are the common causes for this code in these vehicles? I've heard it could be an O2 sensor, but also the catalytic converter itself, which sounds expensive. Any advice on how to diagnose it further before I start throwing parts at it?

Thanks for any help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 5:31 AM

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ClearTheCode admin answer:

The P0420 code on your 2020 Mitsubishi Outlander Sport ES indicates that the catalytic converter's efficiency is below the required threshold. This doesn't always mean the catalytic converter is bad, but it's a strong possibility. Here's a breakdown of common causes and diagnostic steps:

Common Causes for P0420:

  1. Faulty Oxygen (O2) Sensors: Both the upstream (pre-cat) and downstream (post-cat) O2 sensors play a crucial role. A failing downstream O2 sensor might incorrectly report low catalytic efficiency, or a failing upstream sensor could cause the engine to run rich, damaging the converter over time.
  2. Exhaust Leaks: Leaks in the exhaust system before or near the catalytic converter can allow unmetered air in, affecting O2 sensor readings and triggering the code.
  3. Damaged Catalytic Converter: The most direct cause. Over time, catalytic converters can become clogged or damaged internally due to various factors like engine misfires, excessive oil consumption, or coolant leaks.
  4. Engine Misfires or Rich Fuel Mixture: Persistent misfires or an engine running too rich can send unburnt fuel into the catalytic converter, causing it to overheat and fail prematurely.
  5. Faulty Engine Coolant Temperature (ECT) Sensor: An incorrect reading from this sensor can lead to a rich fuel mixture, impacting the converter.

Diagnostic Steps You Can Take (or Ask a Shop To Perform):

  1. Check for Exhaust Leaks: Visually inspect the exhaust system for any cracks, holes, or loose connections, especially around the manifold and catalytic converter.
  2. Monitor O2 Sensor Readings: Using an OBD-II scan tool with live data capabilities, monitor the voltage readings of both upstream and downstream O2 sensors. The upstream sensor should fluctuate rapidly, while the downstream sensor should show a relatively steady, lower voltage. If the downstream sensor mirrors the upstream, it suggests the converter isn't doing its job.
  3. Inspect for Engine Issues: Look for signs of misfires (rough idle, hesitation), excessive oil consumption, or coolant leaks, as these can contribute to converter failure.

When to See a Shop:

If you're not comfortable performing the diagnostic steps yourself, or if the O2 sensor data doesn't clearly point to a sensor issue, it's best to take your Outlander Sport to a qualified mechanic. They have specialized tools and experience to accurately diagnose the root cause of the P0420 code. Ignoring this code can lead to decreased fuel economy, reduced engine performance, and potentially more costly repairs down the line if the catalytic converter fails completely.

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