P0420 Ford Explorer
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 Ford Explorer 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 Ford Explorer.
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 Ford Explorer.
Common Causes on Ford Explorer
- Catalytic converter failure on 2.3L EcoBoost or 3.5L EcoBoost (Explorer/Edge)
- Downstream O2 sensor wiring harness chafing against underbody causing signal issues
- Exhaust manifold/turbo gasket leak diluting exhaust readings on 2.3L EcoBoost
- Catalyst contamination from turbo oil seal leak on high-mileage EcoBoost engines
- Fuel injector leak enriching one cylinder and overloading catalyst with unburned fuel
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 Ford Explorer
On Explorer AWD models, catalyst access is difficult due to transfer case positioning.