P0400moderate severityPowertrain
Exhaust Gas Recirculation (EGR) Flow
on 2005-2011 Audi A6 4.2L V8 · AWD
What this code means
The EGR system flow is not within expected parameters.
Symptoms
- Check Engine Light
- Rough idle
- Pinging/knocking
- Increased NOx emissions
- Failed emissions test
Common causes
- Clogged EGR passages
- Faulty EGR valve
- EGR valve stuck closed
- Carbon buildup
- Vacuum line issue
Diagnostic steps
- Test EGR valve operation
- Check EGR passages for carbon buildup
- Test vacuum supply to EGR valve
- Monitor EGR position with scan tool
- Inspect EGR tube
Repair notes
Clean EGR valve and passages. Replace EGR valve if stuck or faulty.
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