P0509: Idle Air Control System Circuit High
The engine control unit has detected a high signal in the idle air control valve circuit.
Normal risk
Usually drivable, but idle may be too high or unstable. Have it checked soon.
Short answer · Can I keep driving with P0509?
Usually yes, but the idle may be too high or hunt up and down. Have the valve and wiring checked soon.
What does P0509 mean?
Code P0509 is set on engines with a separate idle air control valve. The control unit sees a voltage above the expected range in its circuit, from a short to voltage, an open ground or a failed valve.
What the car does when the code is set
The check engine light comes on. The idle speed may be too high, too low or unstable, and the engine may stall when you come to a stop or switch on consumers such as the air conditioning.
How the system works
At idle the throttle is almost closed, so the engine control unit controls the idle speed by letting a precise amount of air past it. Older engines use a separate idle air control valve; modern engines move the electronic throttle plate itself. The control unit adjusts the air and ignition timing to hold a target idle speed that changes with temperature and loads such as air conditioning. If the real idle speed stays away from the target despite full correction, it stores an idle control code.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Dirty throttle bodyCarbon around the throttle plate changes the idle air flow.
- Vacuum leakExtra air raises the idle speed.
- Faulty idle control valve or electronic throttleThe control part sticks.
- Throttle not relearnedAfter a battery disconnection or cleaning, some engines need an idle relearn.
How it is diagnosed
- Check for vacuum leaksListen for hissing and smoke-test the intake.
- Clean the throttle bodyUse a dedicated cleaner. Follow the service data for electronic throttles.
- Run an idle relearnMany engines need a relearn procedure with a scan tool after cleaning.
- Test the control partCheck the idle valve or throttle motor operation.
Questions about P0509
Can cleaning the throttle body fix P0509?
Not usually. P0509 is electrical; dirt causes idle speed codes such as P0506 or P0507.
Can low battery voltage affect idle?
Yes, some systems change idle speed to charge the battery. Check the battery first if idle problems appear with electrical faults.
Should idle relearn be done after cleaning?
Yes, many engines need it. Without it the idle may be unstable until the engine relearns by itself.