P0420 Nissan Sentra
Catalyst System Efficiency Below Threshold (Bank 1) · Powertrain Code · Medium · Repair: Moderate
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 Nissan Sentra 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 Nissan Sentra.
This repair is rated moderate, meaning some experience and specialty tools required.
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 Nissan Sentra.
Common Causes on Nissan Sentra
- Downstream O2 sensor degradation on QR25DE Altima causing false efficiency readings
- Catalytic converter substrate cracking on Altima from exhaust heat cycling
- Exhaust leak at catalytic converter mounting flange from corroded bolts on Sentra
- Upstream O2 sensor slow response causing rich/lean swings that stress catalyst
- Catalytic converter precious metal depletion from high mileage on QR25DE engine
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 Nissan Sentra
Nissan QR25DE is known for reliable catalytic converters but downstream O2 sensors fail frequently.