GMC Sierra
P0421
The P0421 code indicates an issue with the warm-up catalytic converter efficiency in a 2019-Present GMC Sierra T1XX. This typically suggests that the catalytic converter is not functioning properly or that there is an issue with the oxygen sensors.
2 SolutionsP-CodePowertrain
P
Error Code Type
Powertrain
Components
1 affected
Exhaust System
Symptoms
8 common
- •Check engine light illuminated
- •Decreased fuel efficiency
- •Poor acceleration or power loss
- +5 more
Most Affected Generations
1 generation
T1XX
2
Top Affected Components
1 component
Exhaust System
2
Severity Levels
2 levels
medium
1
high
1
Quick Facts
2
Total Solutions
1
Affected Components
8
Common Symptoms
1
Generations
All Solutions
Showing 2 of 2 solutions for error code P0421 across all generations.