P0509Idle Air Control System Circuit High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- CARB requires the idle speed control to be monitored: petrol engines must flag a fault if the target idle speed is not reached within +200/−100 rpm, diesels within ±30 %. Older engines use a separate idle air control valve that bypasses the closed throttle; with electronic throttle the throttle itself controls idle and no separate valve exists. P0509 is the counterpart of P0508: the drive circuit of the idle valve is shorted to positive or shows implausibly many signal edges – Subaru sets it at 225 or more in 64 ms, with the warning lamp at once and limp mode without idle feedback. It only exists on engines with a separate idle valve.
- What you may notice
- Idle too high or fluctuating
- Stalling at idle
- Check engine light on at once
- Likely causes
- LikelyControl wire shorted to positive
- PossibleIdle valve with shorted turns
- PossibleConnector corroded or wet, interference
- RareEngine module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €50 – €350
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.