Engine Error
P0421
Warm Up Catalyst Efficiency Below Threshold (Bank 1)
Severity
Medium
Symptoms
Check engine light, failed emissions
Common Causes
- Failing catalytic converter
- Engine misfire
- Old O2 sensors
- Exhaust leak
How to Fix
- 1 Perform catalytic converter efficiency test
- 2 Replace catalytic converter
- 3 Replace upstream/downstream O2 sensors
- 4 Fix misfire issues
Estimated Repair Cost
USD
$400
–
$2000
Converter: $400 - $1,800; O2 sensors: $200