2018 Outlander Sport ES - P0420 Check Engine Light - Catalyst System Efficiency

ClearTheCode admin answer:

The P0420 code on your 2018 Mitsubishi Outlander Sport ES indicates that the catalytic converter's efficiency is below the required threshold. This doesn't always mean the converter is bad, but it's a common culprit.

Here's a breakdown of common causes and diagnostic steps:

1. Oxygen Sensors: The most frequent cause is a faulty downstream (post-catalytic converter) oxygen sensor. This sensor monitors the efficiency of the catalyst. If it's reading incorrectly, it can trigger the P0420. An upstream O2 sensor issue can also indirectly cause this by affecting the air/fuel mixture.
2. Exhaust Leaks: Leaks in the exhaust system before or near the catalytic converter can allow unmetered air in, confusing the O2 sensors and leading to a false P0420.
3. Catalytic Converter Failure: Over time, catalytic converters can become clogged or degrade, losing their ability to properly convert exhaust gases. This is often due to engine misfires, excessive oil consumption, or rich fuel mixtures.
4. Engine Misfires/Fuel System Issues: Persistent misfires or problems with the fuel injectors can send unburnt fuel into the catalytic converter, causing it to overheat and fail prematurely.

What to do next:

• Inspect for Exhaust Leaks: Visually check your exhaust system for any cracks, holes, or loose connections, especially around the O2 sensors and converter.
• Check O2 Sensor Readings: If you have access to a more advanced scan tool, monitor the live data for both upstream and downstream O2 sensors. The downstream sensor should show a relatively steady voltage compared to the fluctuating upstream sensor if the catalyst is working correctly.
• Consider Fuel System Cleaners: Sometimes, a good quality fuel system cleaner can help if the issue is minor carbon buildup.

When to see a shop:

If after basic inspection you don't find any obvious exhaust leaks, or if you don't have the tools to monitor O2 sensor data, it's best to take your Outlander Sport to a qualified mechanic. They can perform more in-depth diagnostics, including exhaust backpressure tests and specific O2 sensor tests, to accurately pinpoint whether it's a sensor, an exhaust leak, or the catalytic converter itself that needs replacement. Ignoring this code can lead to reduced fuel economy and potentially more expensive repairs down the line.