P0420
Medium SeverityCatalyst System Efficiency Below Threshold (Bank 1)
Emissions System
What Does P0420 Mean?
The catalytic converter on bank 1 is not converting pollutants efficiently. This is one of the most common DTCs with ~200K monthly searches.
Common Fix
Replace catalytic converter or downstream O2 sensor
Estimated Cost (DIY)
$100 - $1,500
Symptoms
- ⚠️Check engine light
- ⚠️Failed emissions test
- ⚠️Slightly reduced fuel economy
- ⚠️Possible rotten egg smell
Common Causes
Worn catalytic converterlikely
Faulty downstream O2 sensorpossible
Exhaust leak near sensorspossible
Engine running rich (damaging the cat)possible
How to Diagnose P0420
- 1Check for O2 sensor codes first — fix those before replacing cat
- 2Compare upstream vs downstream O2 waveforms
- 3Check for exhaust leaks
- 4Monitor fuel trims for rich/lean issues
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Frequently Asked Questions
- Can I drive with P0420?
- Yes, it is safe to drive. There are no driveability symptoms. However, you will fail emissions testing.
- Is P0420 always the catalytic converter?
- No — about 30% of the time it is actually a bad downstream O2 sensor giving false readings. Replace the sensor first ($25-$80) before spending $500+ on a cat.