2022 Mirage G4 Poor Fuel Economy & Check Engine Light (P0420)

ClearTheCode admin answer:

The P0420 diagnostic trouble code on your 2022 Mitsubishi Mirage G4 indicates "Catalyst System Efficiency Below Threshold (Bank 1)". This means the oxygen sensor downstream of the catalytic converter is reporting similar readings to the upstream sensor, suggesting the converter isn't efficiently cleaning the exhaust gases. While it's surprising on a newer vehicle, there are several reasons this can occur.

Potential Causes for P0420 and Poor Fuel Economy:

1. Failing Catalytic Converter: This is the most direct interpretation of the code. The internal matrix of the converter can degrade, become clogged, or be damaged, reducing its efficiency. While less common at 30,000 miles, it's not impossible, especially if the engine has been running rich or burning oil previously.
2. Faulty Oxygen Sensors:
   • Downstream (Post-Cat) O2 Sensor: A failing downstream O2 sensor can provide incorrect readings, falsely indicating a P0420. This is a common misdiagnosis for a P0420.
   • Upstream (Pre-Cat) O2 Sensor: A faulty upstream O2 sensor can cause the engine to run rich or lean, which can, in turn, damage the catalytic converter or cause it to operate inefficiently.
3. Exhaust Leaks: Leaks in the exhaust system before or between the O2 sensors can allow unmetered air in, skewing O2 sensor readings and triggering a P0420.
4. Engine Misfires or Rich Running Condition: Persistent engine misfires or an overly rich air-fuel mixture (e.g., due to faulty spark plugs, injectors, or a MAF sensor) can dump unburnt fuel into the catalytic converter, causing it to overheat and fail prematurely.

Troubleshooting Steps:

1. Scan for Other Codes: Check if any other codes are present, especially misfire codes (P030x) or fuel trim codes (P0171, P0172). These could point to the root cause of the P0420.
2. Inspect Exhaust System: Visually check for any exhaust leaks, especially around the O2 sensors and catalytic converter.
3. Monitor O2 Sensor Data: A professional scan tool can monitor live data from both upstream and downstream O2 sensors. The downstream sensor should show a relatively flat line compared to the oscillating upstream sensor if the converter is working correctly.
4. Check for Engine Performance Issues: Ensure the engine is running smoothly, without misfires, rough idle, or excessive smoke from the exhaust.

When to See a Shop:

Given the P0420 code and the drop in fuel economy, it's advisable to have your Mirage G4 inspected by a mechanic. They can accurately diagnose whether it's a failing O2 sensor or the catalytic converter itself. While a bad O2 sensor is a simpler fix, a failing catalytic converter is expensive. Driving with a P0420 can lead to further damage to the exhaust system and can cause your vehicle to fail emissions testing. Addressing any underlying engine issues (like misfires) is critical to prevent future catalytic converter problems.