P0420Códigos OBD-II
OBD-II P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
The downstream oxygen sensor sees too little catalyst work on bank 1.
Gravedad: mediumCatalytic converter equipped gasoline engines
Significado técnico
Post-cat O2 sensor voltage mimics the pre-cat signal — the converter has lost its oxygen storage capacity.
Síntomas
- Check-engine light
- Failed emissions test
- Slight power loss and fuel smell
3 causas más probables
- 1Ageing, overheated or poisoned catalytic converter
- 2Exhaust leak ahead of the downstream sensor
- 3Misfire or rich mixture damaged the catalyst earlier
Cómo solucionarlo
- 1Repair any misfire or rich-mixture fault first — see P0300 and P0171.
- 2Inspect the exhaust for leaks ahead of the post-cat oxygen sensor.
- 3Compare pre-cat and post-cat O2 waveforms at a steady 2 500 rpm.
- 4Replace the converter only after every upstream fault is fixed, otherwise it fails again.
ELM327 / OBD-II
ATZ # ELM327 reset ATE0 0101 # fuel trim / DTC count 03 # read stored DTCs 04 # clear DTCs after the repair
Preguntas frecuentes
- Can I drive with P0420?
- Yes, but you will fail the emissions test and the converter can overheat if the root cause is a misfire.
- Will an O2 sensor spacer fix it?
- It only masks the symptom and is illegal for road use in most countries — fix the root cause instead.
Copias: 620 · Visitas: 1840 · Actualizado: 18 sept 2026
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