P0509 code meaning
Idle Air Control System Circuit High
Powertrain system Generic code P05xx codes Check engine light
What P0509 means
CARB requires the idle speed control to be monitored: gasoline 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.
P0509 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P0509
- Idle too high or fluctuating
- Stalling at idle
- Check engine light on at once
Common causes of P0509
- Most likelyControl wire shorted to positive
- PossibleIdle valve with shorted turns
- PossibleConnector corroded or wet, interference
- Less likelyEngine module faulty
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Idle air control valveValve wiringEngine control module
What each part of P0509 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 5 | Area: vehicle speed, idle control and auxiliary inputs. |
| 09 | The exact fault in that area. |
What to do next
- Note P0509 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0509
What does P0509 mean?
P0509 means "Idle Air Control System Circuit High". CARB requires the idle speed control to be monitored: gasoline engines must flag a fault if the target idle speed is not reached within +200/−100 rpm, diesels within ±30 %. It is a generic code, so it means the same on every make.
Is it safe to drive with P0509?
Drive gently and only to a workshop. With P0509 the car can go into limp mode (reduced power) to protect itself.
Will P0509 fail an emissions test?
Yes, it can. P0509 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0509 to fix?
Difficulty: medium. A typical repair takes 0.5 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0509 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.