P0421moderate severityPowertrain
Warm Up Catalyst Efficiency Below Threshold Bank 1
on 2004-2006 Audi A8 4.2L V8 · AWD
What this code 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
Repair notes
Replace warm-up catalyst if confirmed failed.
Other Powertrain codes on this vehicle
P0001
Fuel Volume Regulator Control Circuit/Open
P0002
Fuel Volume Regulator Control Circuit Range/Performance
P0003
Fuel Volume Regulator Control Circuit Low
P0004
Fuel Volume Regulator Control Circuit High
P0010
Intake Camshaft Position Actuator Circuit/Open Bank 1
P0011
Intake Camshaft Position Timing Over-Advanced Bank 1
P0012
Intake Camshaft Position Timing Over-Retarded Bank 1
P0013
Exhaust Camshaft Position Actuator Circuit/Open Bank 1
P0014
Exhaust Camshaft Position Timing Over-Advanced Bank 1
P0015
Exhaust Camshaft Position Timing Over-Retarded Bank 1