P0420 Catalyst System Efficiency Below Threshold (Bank 1)
What does P0420 mean?
P0420 — Catalyst System Efficiency Below Threshold (Bank 1) (bank 1). This is an OBD-II diagnostic trouble code a scanner retrieves when your vehicle's computer detects catalyst system efficiency below threshold (bank 1) on bank 1. The level of risk depends on how the vehicle is currently running and how long you've been driving with the light on.
Common causes of P0420
- Failing catalytic converter
- Oxygen sensor failure (upstream or downstream)
- Exhaust leak before the O2 sensor
- Engine misfire dumping raw fuel into the converter
- Oil or coolant burning from engine issues
How to fix P0420
- Test upstream and downstream oxygen sensor switching
- Check for exhaust leaks with a smoke test
- Monitor fuel trim values using an OBD-II scanner
- Diagnose and fix misfires before condemning the converter
- Replace the catalytic converter if efficiency remains below threshold after fixing other issues