P3891moderate severityPowertrain
Ammonia Slip Catalyst Efficiency Low
on 2012 Audi A3 2.0L TDI I4 · FWD
What this code means
The ammonia slip catalyst (ASC) downstream of the SCR is not effectively converting excess ammonia to nitrogen. Ammonia is escaping to atmosphere.
Symptoms
- Ammonia smell from exhaust
- Check Engine Light
- Failed emissions test
- Eye and throat irritation near exhaust
- White haze from tailpipe
Common causes
- ASC catalyst degradation
- SCR over-dosing DEF causing excess ammonia
- ASC contamination
- Exhaust temperature too low for ASC
- ASC catalyst physically damaged
Diagnostic steps
- Measure ammonia levels downstream of ASC
- Check DEF dosing rates for over-dosing
- Monitor SCR and ASC temperatures
- Inspect ASC for physical damage
- Test downstream NOx and NH3 sensors
Repair notes
Replace ammonia slip catalyst. Correct any DEF over-dosing issue. Inspect SCR system calibration.
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