P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
Reviewed2 sources
2 sources
- Official data (NHTSA)official_feed
- Official data (NHTSA)official_feed
What P0420 means
Caution — monitor closely
The rear oxygen sensor shows the catalytic converter is no longer effectively burning off unburned fuel residue — a downstream signal, not always the root cause itself.
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General information, not a diagnosis or repair instruction. Always confirm with a qualified mechanic for your specific vehicle.
Common causes
- Natural catalyst wear
- Aged or failing catalytic converter
- Faulty rear oxygen sensor
- Faulty upstream oxygen sensor
- Exhaust leak or burn-through
Symptoms
- Повышенный расход топлива
- Sulfur smell from the exhaust
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