2019 Outlander SEL Check Engine Light - P0420 code

ClearTheCode admin answer:

The P0420 code, 'Catalyst System Efficiency Below Threshold (Bank 1),' is a common diagnostic trouble code (DTC) that indicates your vehicle's catalytic converter is not performing as efficiently as it should be. While it often points to a failing catalytic converter, there are other possibilities that are less costly.

Here's a breakdown of potential causes and troubleshooting steps:

1. Failing Catalytic Converter:

   • Cause: Over time, catalytic converters can degrade internally due to age, contamination (e.g., from excessive oil burning or coolant leaks), or physical damage. When it can no longer convert harmful exhaust gases efficiently, the post-catalytic converter (downstream) oxygen sensor will report similar readings to the pre-catalytic converter (upstream) sensor, triggering P0420.
   • Action: This is the most expensive fix, requiring replacement of the catalytic converter. Before replacing, confirm other potential causes are ruled out.

2. Faulty Oxygen (O2) Sensors:

   • Cause: While P0420 specifically points to catalyst efficiency, a failing downstream O2 sensor (the one after the catalytic converter) can sometimes give inaccurate readings, falsely indicating a P0420. Less commonly, a failing upstream O2 sensor can cause the engine to run rich, leading to premature catalytic converter failure.
   • Action: A mechanic can monitor live data from both O2 sensors using a professional scanner. Look for erratic readings from the downstream sensor or if both sensors are mirroring each other's waveforms.

3. Exhaust Leaks:

   • Cause: An exhaust leak before or near the downstream O2 sensor can allow unmetered air into the exhaust stream, skewing sensor readings and potentially triggering P0420.
   • Action: Visually inspect the exhaust system for cracks, holes, or loose connections, especially around the catalytic converter and O2 sensor bungs. Listen for hissing or puffing sounds.

4. Engine Misfires or Fuel System Issues:

   • Cause: Persistent engine misfires, running too rich/lean, or excessive oil consumption can send unburnt fuel or contaminants into the catalytic converter, damaging it over time and leading to P0420.
   • Action: Check for other pending or active codes related to misfires (P030x) or fuel trim issues (P0171, P0172).

When to see a shop: Since you have a specific code, it's best to take your Outlander to a reputable mechanic or Mitsubishi dealership. They can perform a thorough diagnostic, including:

• Live Data Analysis: To compare upstream and downstream O2 sensor readings.
• Exhaust Backpressure Test: To check for a clogged catalytic converter.
• Visual Inspection: For exhaust leaks or physical damage.

While the car drives fine now, ignoring a P0420 can lead to decreased fuel economy and, eventually, more severe issues if the catalytic converter becomes completely clogged.