Understanding the P0420 Code
The P0420 code means "Catalyst System Efficiency Below Threshold (Bank 1)". This means your car's computer has detected that the catalytic converter is not functioning as efficiently as it should be.
Common Causes
- Faulty oxygen (O2) sensor (front or rear)
- Exhaust leak near the catalytic converter
- Failed or degraded catalytic converter
- Engine misfires running rich/lean
Don't Just Replace the Converter!
Mechanics love to quote $1,000+ for a new catalytic converter when they see a P0420 code. However, in many cases, the issue is actually a faulty downstream O2 sensor. An O2 sensor costs around $50 on Amazon and takes 20 minutes to replace.
Recommended O2 Sensor Socket Tool
If you're going to replace the sensor yourself, you will need a special slotted socket. View O2 Sensor Tools on Amazon.