P0420Medium severity
Catalyst System Efficiency Below Threshold (Bank 1)
The catalytic converter on bank 1 is not operating as efficiently as expected, based on the downstream oxygen sensor readings.
Common causes
- Worn catalytic converter
- Faulty downstream oxygen sensor
- Exhaust leak
- Engine running rich or lean
Possible symptoms
- Check Engine Light on
- Reduced fuel economy
- Sulfur smell from exhaust
- Failed emissions test
Code breakdown
P — System
Powertrain (engine, transmission, fuel & emissions)
0 — Type
Generic (SAE standard, common to all manufacturers)
4 — Subsystem
Auxiliary emission controls