P3891moderate severityPowertrain
Ammonia Slip Catalyst Efficiency Low
on 2013 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.
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