P0420 – Catalyst System Efficiency Below Threshold (Bank 1)
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Symptoms
- Check Engine Light illuminated
- Reduced engine performance
- Increased fuel consumption
- Strong sulfur smell
- Failed emissions test
- Possible rattling noise
Causes
- Worn or damaged catalytic converter
- Exhaust leak before the converter
- Faulty oxygen sensors
- Long-term rich or lean fuel mixture
- Engine misfires
- Overheated or damaged converter
- Contaminated fuel or oil burning entering the exhaust
Diagnosis
- Check O2 sensor readings
- Inspect catalytic converter temperature
- Check fuel trims
- Inspect for exhaust leaks
Repair
- Replace catalytic converter
- Fix exhaust leaks
- Replace faulty O2 sensors
- Correct fuel mixture issues
- Repair misfires to prevent future damage