What does P0420 mean?
P0420 indicates that the engine control system has determined that the catalytic converter's efficiency is below the expected threshold for the monitored bank.
The exact diagnostic strategy varies by vehicle, but oxygen-sensor signals are commonly used by the control system to evaluate catalyst performance.
Possible symptoms
- Check engine light
- Reduced performance in some cases
- Poor fuel economy in some cases
- Exhaust-related symptoms
- Sometimes no obvious drivability symptoms at all
Possible causes
- Worn or damaged catalytic converter
- Engine misfire
- Incorrect air-fuel mixture
- Exhaust leak
- Faulty oxygen or air-fuel sensor
- Sensor wiring problems
- Oil or coolant contamination of the exhaust system
- Previous engine problems that damaged the catalyst
Why diagnosis is important
Replacing the catalytic converter immediately may not solve the underlying problem.
If an engine is misfiring or running with an incorrect air-fuel mixture, the new catalytic converter may also be damaged.
- Check for other stored trouble codes
- Inspect the exhaust system for leaks
- Review oxygen-sensor or air-fuel-sensor data
- Check engine misfire information
- Investigate fuel-trim readings
- Verify the engine is operating correctly
Is P0420 always a bad catalytic converter?
No. P0420 indicates a catalyst-efficiency monitoring problem, but the root cause must be established before replacing expensive components.