2020 Outlander Sport P0420 Code - CEL On, What's Next?

ClearTheCode admin answer:

The P0420 code, 'Catalyst System Efficiency Below Threshold (Bank 1)', indicates that your vehicle's catalytic converter is not operating as efficiently as it should be. While your 2020 Mitsubishi Outlander Sport ES might seem to drive normally, this issue can lead to increased emissions and potentially fail an emissions test.

Here are the common causes and diagnostic steps:

1. Oxygen Sensors (O2 Sensors):

   • Cause: Faulty upstream or downstream O2 sensors can send incorrect readings to the engine control unit (ECU), leading it to falsely believe the catalytic converter is inefficient.
   • Diagnosis: A scan tool capable of live data can show O2 sensor readings. The downstream sensor should have a relatively steady voltage compared to the fluctuating upstream sensor. If both fluctuate similarly, it might indicate a failing catalytic converter or a faulty downstream O2 sensor.

2. Exhaust Leaks:

   • Cause: Leaks in the exhaust system before the downstream O2 sensor can allow unmetered air in, affecting O2 sensor readings and triggering the P0420 code.
   • Diagnosis: Visually inspect the exhaust system for cracks, holes, or loose connections. Listen for hissing sounds, especially when the engine is cold.

3. Catalytic Converter Failure:

   • Cause: Over time, catalytic converters can become clogged or degrade internally, losing their ability to efficiently convert harmful exhaust gases. This is the most serious and often most expensive cause.
   • Diagnosis: If O2 sensors and exhaust leaks are ruled out, the catalytic converter itself is likely the culprit. A backpressure test can confirm a clogged converter.

4. Engine Misfires or Rich/Lean Conditions:

   • Cause: Persistent engine misfires or an engine running too rich or too lean can damage the catalytic converter over time.
   • Diagnosis: Check for other pending or active engine codes (e.g., P030x for misfires, fuel trim codes). Address these underlying issues first.

When to see a shop: If you've checked for obvious exhaust leaks and confirmed your O2 sensors appear to be functioning correctly with a live data scanner, it's advisable to take your 2020 Outlander Sport to a qualified mechanic. Diagnosing a P0420 can be complex, and a professional can perform more advanced tests like backpressure checks or smoke tests to accurately pinpoint the problem and recommend the correct repair, whether it's replacing an O2 sensor or the catalytic converter.