P0420 Toyota Highlander
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 Toyota Highlander 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 Toyota Highlander.
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 Toyota Highlander.
Common Causes on Toyota Highlander
- Downstream O2 sensor failure on RAV4/Highlander causing false catalyst efficiency code
- Catalytic converter gradual degradation on 3.5L V6 Highlander from high mileage
- Exhaust leak at catalytic converter-to-exhaust pipe flange from corrosion on RAV4
- Upstream O2 sensor response delay causing rich/lean oscillation degrading catalyst
- Secondary air injection system malfunction on V6 models introducing moisture into catalyst
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 Toyota Highlander
Toyota catalytic converters are high-theft targets due to high precious metal content; consider cat shields.