P0420 GMC Sierra 1500
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 GMC Sierra 1500 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 GMC Sierra 1500.
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 GMC Sierra 1500.
Common Causes on GMC Sierra 1500
- Catalytic converter substrate deterioration from AFM oil consumption on 5.3L V8
- Downstream O2 sensor (bank 1 sensor 2) reading inaccurately due to contamination
- Secondary air injection system check valve failure allowing moisture into catalyst
- Exhaust manifold bolt failure causing exhaust leak upstream of catalyst
- Catalytic converter thermal damage from prolonged misfire on AFM-deactivated cylinders
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 GMC Sierra 1500
Always fix misfires before replacing catalytic converter; unburned fuel destroys new cats.