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P0420: Catalyst System Efficiency Below Threshold
The computer is seeing the catalytic converter working below the expected efficiency range, usually based on downstream oxygen sensor behavior.
Common causes
- Worn catalytic converter
- Exhaust leak before or near the converter
- Lazy oxygen sensor
- Rich or lean running condition
This free page explains the code. A Shop Brief adapts the next steps to your year, make, model, and full code stack.
Related codes:
P0133P0171P2096 Pricing Sample brief