P0420 Honda Civic
Catalyst System Efficiency Below Threshold (Bank 1) · Powertrain Code · Medium · Repair: Easy
What is P0420?
The catalytic converter on bank 1 is not operating efficiently. The downstream O2 sensor readings indicate the catalyst is not converting exhaust gases properly. May need catalyst replacement or could be caused by a failed O2 sensor.
When the Honda Civic triggers code P0420 (catalyst system efficiency below threshold (bank 1)), it typically points to an issue in the powertrain system. Below you'll find the most common causes, symptoms, and repair options specific to the Honda Civic.
This repair is rated easy, meaning basic tools, no lift needed.
Diagnostic data on this page is compiled from publicly available repair resources and technical service bulletins. Always have a qualified mechanic confirm the diagnosis before authorizing repairs on your Honda Civic.
Common Causes on Honda Civic
- Downstream O2 sensor (secondary) reading inaccurately on Civic/Accord over 80k miles
- Catalytic converter gradual efficiency loss on 1.5L turbo from direct injection soot
- Exhaust leak at turbo downpipe connection on 1.5L turbo Civic/Accord
- Catalyst contamination from fuel dilution of oil on 1.5T engines (cold climate issue)
- Upstream O2 sensor slow response causing inefficient fuel control and catalyst stress
Symptoms
- Check engine light illuminated steadily
- Reduced fuel economy compared to normal
- Sulfur or rotten egg smell from exhaust
- Failed emissions inspection or smog test
- Slight loss of power at highway speeds
- Rough or uneven exhaust note
Notes for Honda Civic
Honda O2 sensors are the most common cause of P0420; replace sensors before committing to cat replacement.