2018 Outlander Sport P0420 Code - What's Next?

ClearTheCode admin answer:

1. Understanding P0420: This code indicates that the catalytic converter's efficiency for Bank 1 (the only bank on most 4-cylinder engines like in your Outlander Sport) is below the required threshold. It doesn't necessarily mean the catalytic converter is bad, but rather that the exhaust gases coming out of it aren't as clean as they should be.

2. Common Causes: While a failing catalytic converter is a possibility, several other issues can trigger a P0420 code:

   • Oxygen Sensors: Faulty upstream or downstream O2 sensors can send incorrect readings, tricking the ECU into thinking the catalyst is inefficient.
   • Exhaust Leaks: Leaks before or near the O2 sensors can introduce ambient air, skewing readings and affecting catalyst monitoring.
   • Engine Misfires/Running Rich/Lean: Persistent engine issues that cause unburnt fuel or excessive oxygen to enter the exhaust can damage the catalyst over time or trigger the code.
   • Catalytic Converter Failure: Over time, the catalyst material can degrade or become contaminated, reducing its efficiency.

3. Initial Troubleshooting Steps: Before jumping to conclusions, here are a few things you can check:

   • Check for Exhaust Leaks: Visually inspect the exhaust system for any cracks, holes, or loose connections, especially around the O2 sensors. Listen for any hissing sounds.
   • Inspect O2 Sensors: While difficult without specialized tools, sometimes a visual inspection can reveal damaged wiring or heavily fouled sensors. A scan tool capable of live data can show O2 sensor readings, which can help diagnose if they are switching correctly.
   • Monitor Fuel Trims: Using a scan tool, check short-term (STFT) and long-term (LTFT) fuel trims. High positive or negative trims can indicate an underlying engine issue affecting exhaust composition.
   • Perform a Drive Cycle: Sometimes, after clearing the code, a specific drive cycle can help the ECU re-evaluate the catalyst efficiency. Consult your owner's manual or an online resource for the specific Mitsubishi drive cycle.

4. When to See a Shop: If after checking for obvious issues like exhaust leaks, the code persists, or if you notice significant performance issues (reduced power, poor fuel economy, strong exhaust smell), it's best to consult a professional mechanic. They have specialized diagnostic tools (like exhaust gas analyzers) to accurately test the catalytic converter and O2 sensors, preventing unnecessary parts replacement. Addressing the root cause is crucial to avoid recurring issues and ensure your vehicle passes emissions tests.