P0420 Toyota 4Runner
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 4Runner 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 4Runner.
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 4Runner.
Common Causes on Toyota 4Runner
- Catalytic converter degradation from towing and heavy-duty use on 3.5L V6 Tundra/Tacoma
- Downstream O2 sensor contamination from rust particles in exhaust on older 4Runner
- Secondary air injection system failure on 4.0L V6 4Runner causing P0420 with P0418
- Exhaust manifold gasket leak from thermal cycling during tow-and-release driving patterns
- Catalytic converter physical damage from off-road impacts on Tacoma/4Runner
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 4Runner
Toyota truck catalytic converters are extremely high-theft items; install anti-theft cat shields.