P0420
Moderate
Powertrain
Catalyst System Efficiency Below Threshold (Bank 1)
Common Causes
Degraded or failed catalytic converter
Failed downstream oxygen sensor
Exhaust leak before or at catalytic converter
Engine burning oil or coolant
Incorrect fuel type or contaminated fuel
Common Symptoms
Common Repairs
#1 Replace Downstream O2 Sensor
30–60 min
$20 – $80
Easy
The downstream oxygen sensor monitors catalytic converter efficiency. A failed sensor triggers P0420 even when the cat is functional.
Parts Needed
Downstream O2 Sensor
#2 Replace Catalytic Converter
1–2 hours
$150 – $600
Moderate
If the O2 sensor tests good, the catalytic converter itself has likely degraded and needs replacement.
Parts Needed
Catalytic Converter
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Quick Info
| Code | P0420 |
| System | Powertrain |
| Severity | Moderate |
| Repairs | 2 known |
Disclaimer
This information is for diagnostic guidance only. Always verify repairs with a qualified technician before performing safety-critical work.
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