A steady check engine light on your fuel-efficient 2022 Mitsubishi Mirage with a P0420 code indicates the catalytic converter isn't cleaning exhaust gases as effectively as it should. While this code often points to converter failure, several other issues can trigger it—and some are far less expensive to repair than a new catalytic converter.
Understanding P0420
P0420 means "Catalyst System Efficiency Below Threshold (Bank 1)." The Mirage's three-cylinder engine has one bank, so this code covers the entire exhaust system. The engine computer compares readings from the oxygen sensors before and after the catalytic converter. When the readings are too similar, the converter isn't doing its job.
Symptoms Accompanying P0420
- Steady check engine light
- Possible slight decrease in fuel economy
- Possible sulfur (rotten egg) smell from exhaust
- Usually no noticeable drivability issues
- May fail emissions inspection
Common Causes of P0420 in the Mirage
Worn Catalytic Converter
Catalytic converters wear out over time. The Mirage's small engine doesn't stress the converter heavily, but age and mileage still take their toll.
Solution: Catalytic converter replacement if truly worn.
Faulty Oxygen Sensors
Before replacing the expensive converter, check the O2 sensors. A failing downstream sensor can falsely trigger P0420 by reporting incorrect values.
Solutions:
- Test O2 sensor operation with scan tool
- Replace downstream sensor if response is sluggish or reading incorrect
Exhaust Leaks
Exhaust leaks before the oxygen sensors allow fresh air to reach the sensors, skewing their readings.
Solutions:
- Inspect exhaust manifold and connections
- Repair any leaks found
Engine Misfires
Misfires send unburned fuel into the converter, damaging it over time. If the Mirage has had ongoing misfire issues, the converter may be damaged as a result.
Solution: Address any misfire codes before or alongside converter replacement.
Fuel Mixture Issues
Running too rich or too lean damages the converter. Check for fuel system codes that might indicate mixture problems.
Solution: Address any fuel system issues.
Oil or Coolant Contamination
Oil or coolant entering the exhaust (from internal engine problems) can poison the catalytic converter.
Solutions:
- Check for signs of oil or coolant consumption
- Address internal engine issues if present
Diagnosis Steps
- Check for other codes: Misfire or fuel system codes may indicate root causes
- Test O2 sensors: Compare upstream and downstream sensor readings with scan tool
- Check for exhaust leaks: Visual and auditory inspection
- Check for misfires: Even intermittent misfires damage converters
- Evaluate converter: If sensors and exhaust are fine, converter is likely failed
Repair Costs
- Oxygen sensor (downstream): $150-$300
- Exhaust leak repair: $100-$400
- Catalytic converter (aftermarket): $400-$900
- Catalytic converter (OEM): $800-$1,500
- Professional diagnosis: $100-$150
The 2022 Mirage has federal emissions warranty coverage (8 years/80,000 miles) on the catalytic converter and related components, separate from the bumper-to-bumper warranty.