P0421Lexus UxYears: 1996 - 2026
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Reviewed by ASE Certified Technician
What does this mean?
Code P0421 indicates the Bank 1 catalytic converter is not reaching proper operating efficiency during the warm-up period.
Disclaimer: This diagnostic code is part of the standard OBD2 system and can apply to Lexus Ux models manufactured from 1996 to 2026. However, please note that the presence of this specific code depends on your vehicle's exact engine configuration, options, and production year.
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Common Causes
Failing or clogged catalytic converter
Faulty oxygen (O2) sensor
Exhaust leak before oxygen sensor
Worn or fouled spark plugs
Symptoms to Watch For
- Check Engine Light is illuminated
- Failed emissions test
- Rotten egg smell (sulfur) from exhaust
- Decreased fuel economy