P0420 is the most-searched OBD-II code in the United States. It stands for "Catalyst System Efficiency Below Threshold (Bank 1)." Here's everything you need to know before calling a shop.
What P0420 means
Your ECU compares readings from the upstream and downstream oxygen sensors on Bank 1. When the downstream sensor's readings look too similar to the upstream sensor — meaning the catalytic converter isn't cleaning exhaust gases — P0420 is stored.
Is it serious?
Medium urgency. Safe to drive short-term but you'll fail an emissions test. If the root cause is engine misfires contaminating the converter, damage compounds over time.
The three most common causes
- Worn catalytic converter — Most common over 100k miles. Only fix is replacement ($400–$2,200).
- Failed O2 sensor — A faulty upstream sensor triggers a false P0420. Much cheaper: $120–$300.
- Exhaust leak — Fresh oxygen near the downstream sensor causes incorrect readings.
Repair costs
- O2 sensor: $120–$300
- Aftermarket catalytic converter: $400–$900
- OEM catalytic converter: $800–$2,200
California/CARB states: Require CARB-compliant converters only — verify before purchasing any replacement part.
See the full P0420 code page for causes ranked by likelihood and a mechanic finder.