P0420 GMC Terrain
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 Terrain 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 Terrain.
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 Terrain.
Common Causes on GMC Terrain
- Catalytic converter internal substrate failure on 1.5L turbo Equinox/Terrain
- Downstream O2 sensor slow response time on 2.0L turbo engine
- Exhaust flex pipe crack between turbo downpipe and catalytic converter causing leak
- Catalyst efficiency loss from oil consumption fouling on 2.4L Ecotec engines
- ECM software calibration mismatch causing false P0420 on certain model years
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 Terrain
On turbo models, the catalytic converter is close-coupled to the turbo and subject to extreme heat.