Diagnostic Trouble Code
P0420 — Catalyst System Efficiency Below Threshold Bank 1
Catalyst System Efficiency Below Threshold Bank 1
Definition
The catalytic converter on Bank 1 is not operating at expected efficiency. The ECM compares upstream and downstream O2 sensor signals — when they mirror each other too closely, the catalyst is no longer converting pollutants effectively.
Common Causes
- Worn or poisoned catalytic converter
- Exhaust leak before the rear O2 sensor
- Faulty downstream O2 sensor
- Engine misfire damaging the catalyst over time
- Oil or coolant contamination in exhaust