P0505moderate severityPowertrain
Idle Air Control System
on 2008 Audi S5 4.2L V8 · AWD
What this code means
Malfunction in the idle air control (IAC) system.
Symptoms
- Erratic idle speed
- High idle
- Low idle
- Stalling at idle
- Fluctuating RPM
Common causes
- Faulty IAC valve
- Carbon buildup in IAC or throttle body
- Vacuum leak
- PCM issue
Diagnostic steps
- Clean IAC valve and passages
- Clean throttle body
- Test IAC valve operation
- Check for vacuum leaks
- Monitor idle RPM with scan tool
Repair notes
Clean IAC valve and throttle body first. Replace IAC if cleaning fails.
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