P0420 Ford Bronco
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 Bronco 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 Bronco.
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 Bronco.
Common Causes on Ford Bronco
- Catalytic converter damage from off-road impact on unprotected underbody exhaust
- Downstream O2 sensor connector water intrusion from deep water crossings
- Exhaust flange gasket leak from off-road vibration loosening connections
- Catalyst substrate fracture from repeated off-road impacts and shock loading
- Extended idle during off-road crawling overheating catalyst from rich exhaust mixture
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 Bronco
Aftermarket cat skid plates are strongly recommended for any Bronco used for off-roading.