Code
P0420
Emissions
Catalyst system efficiency below threshold — Bank 1
The downstream O2 sensor sees too little catalyst activity. Confirm the converter is really bad before replacing it.
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Most likely causes
- Worn or clogged catalytic converter
- O2 sensor reading incorrectly
- Exhaust leak
- Misfires or rich mixture burning the catalyst
Symptoms you may notice
- Check engine light
- Usually no drivability symptom
- Failed emission test
Checks to run
- Compare upstream and downstream O2 waveforms
- Inspect exhaust for leaks
- Fix misfires or rich condition first
Tools you may need
OBD-II scanner
Graphing scan tool
Always diagnose before replacing. Codes share symptoms; confirm the cause with measurements before buying parts.
Related codes in Emissions
P0130