2018 Outlander Sport - P0420 Code and Check Engine Light

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. While it often points to a failing catalytic converter, there are other potential causes that should be investigated first.

Here's a breakdown of common causes and steps to diagnose:

1. Exhaust Leaks: Even small leaks in the exhaust system before or near the catalytic converter can cause the O2 sensors to read incorrectly, triggering a P0420. Check for any audible hissing or black sooty spots around exhaust joints or the manifold.
2. Faulty Oxygen Sensors: While less common for P0420 specifically (usually a P0133/P0134 or P0137/P0138 would appear), a sluggish or failing rear O2 sensor (post-catalyst) can sometimes incorrectly report catalyst inefficiency. A professional scan tool can monitor O2 sensor waveforms.
3. Engine Misfires or Rich/Lean Conditions: Persistent misfires (P030x codes), a leaking fuel injector, or other engine issues causing an overly rich or lean fuel mixture can damage the catalytic converter over time or cause it to operate inefficiently. Ensure there are no other active or pending engine codes.
4. Catalytic Converter Failure: Unfortunately, the catalytic converter itself can fail due to age, contamination (from oil or coolant burning), or physical damage. This is often the most expensive repair.

When to See a Shop:

• If you've checked for exhaust leaks and found none, or if you lack the tools to properly diagnose O2 sensor function.
• If the Check Engine Light persists after clearing the code (if it was a temporary glitch).
• If you notice a significant drop in fuel economy, a rotten egg smell from the exhaust, or a noticeable loss of power.
• A qualified technician can perform a backpressure test, monitor live O2 sensor data, and conduct a more thorough inspection to pinpoint the exact cause before recommending a costly catalytic converter replacement.