A P0420 diagnostic trouble code on your 2024 Honda Civic indicates that the catalytic converter efficiency has fallen below the acceptable threshold. The engine control module monitors converter performance by comparing upstream and downstream oxygen sensor readings, and triggers P0420 when efficiency drops significantly.
Understanding P0420
The catalytic converter transforms harmful exhaust gases (hydrocarbons, carbon monoxide, nitrogen oxides) into less harmful emissions. When working properly, the downstream O2 sensor shows a relatively steady voltage compared to the fluctuating upstream sensor. When the converter loses efficiency, downstream sensor readings begin mimicking upstream patterns.
Common Causes of P0420
- Failing Catalytic Converter: Internal substrate deterioration or contamination
- Faulty Oxygen Sensors: Upstream or downstream O2 sensor malfunction
- Exhaust Leaks: Leaks before or after the converter affecting readings
- Engine Misfires: Unburned fuel damages converter over time
- Oil Burning: Internal engine issues contaminating the converter
- Rich/Lean Conditions: Fuel system issues accelerating converter wear