P0421
PowertrainCatalystmoderate severityWarm Up Catalyst Efficiency Below Threshold Bank 1
What P0421 Means
The Bank 1 warm-up catalytic converter is not efficient during the warm-up period.
Symptoms
- Check Engine Light
- Failed emissions test
Common Causes
- Worn catalyst
- Engine running rich during warm-up
- O2 sensor issue
- Exhaust leak
Diagnostic Steps
- Monitor catalyst temps during warm-up
- Compare O2 sensor signals
- Check for exhaust leaks
Where to start: Replace warm-up catalyst if confirmed failed.
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