P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter on bank 1 is not operating as efficiently as expected.
- Applies to
- All OBD-II vehicles (any make or model)
- Est. repair cost
- $200 – $2,200
- Time to fix
- 1-4 hours
What it means
P0420 is triggered when the downstream oxygen sensor readings indicate the catalytic converter is no longer cleaning exhaust gases effectively.
Symptoms
- Check engine light on
- Slight drop in fuel economy
- Rotten-egg smell from exhaust
- Failed emissions test
How to fix it
- 1
Verify oxygen sensors
1 hour1. Read live O2 sensor data. 2. Compare upstream vs downstream signals. 3. Replace a lazy or dead sensor before condemning the converter.
- 2
Replace catalytic converter
2 hours1. Confirm the converter is the fault, not sensors or exhaust leaks. 2. Replace with an OEM-grade converter. 3. Clear the code and complete a drive cycle.
Frequently asked questions
Will P0420 clear itself?
Occasionally after fixing an exhaust leak, but usually the underlying converter or sensor issue must be repaired.
Last updated July 13, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.