OBD-II Code · Fuel & Air
P0509
Idle Air Control Circuit High
Severity
medium
Worth booking. It costs fuel, emissions or comfort until it is fixed.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$80–$400
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
IAC voltage high.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- High idle
Likely causes
- Wiring short to powerMost common
- Failed IAC
Read the full diagnostic procedureP0509 is the high-voltage counterpart to P0508 and means the PCM sees the IAC driver pin sitting at battery voltage when it should be pulled low duri…
P0509 is the high-voltage counterpart to P0508 and means the PCM sees the IAC driver pin sitting at battery voltage when it should be pulled low during commanded operation, indicating an open IAC winding, an open driver circuit, or a short-to-voltage.
Cheapest-first diagnosis
- disconnect the IAC and ohm the solenoid windings, an open coil reads OL (infinite) and confirms an internally broken IAC. With the connector plugged in and the engine running, scope the IAC driver wires, you should see a square-wave PWM signal pulling the line from roughly 12V down to under 1V at the commanded duty cycle.
- A line that stays at 12V means the PCM driver is not switching, but before condemning the PCM, check the IAC ground path, a corroded splice or open ground will mimic a PCM failure.
- Inspect the connector terminals for spread or backed-out pins, a $3 terminal repair fixes a surprising number of P0509 complaints.
- Bench-test the IAC by applying 12V and ground briefly to each coil pair and listening for the pintle click.
- The expensive misdiagnosis is reflashing or replacing the PCM when an open IAC harness wire or a corroded pin is the actual cause.
Vehicle-specific patterns1999 to 2004 GM 4.3L/4.8L/5.3L sets P0509 from open IAC windings, the failure mode is winding fatigue rather than a short.
- 1999to 2004 GM 4.3L/4.8L/5.3L sets P0509 from open IAC windings, the failure mode is winding fatigue rather than a short.
- 1998to 2003 Ford Ranger 3.0L Vulcan sets P0509 from a broken IAC connector pigtail.
- 2000to 2006 Jeep Wrangler TJ 4.0L sets P0509 from a chafed IAC harness at the valve cover edge.
- 1996to 2001 Honda CR-V 2.0L sets P0509 from a failed IAC solenoid winding.
Estimated repair$30 to $200.
Look up another code
P0509 — frequently asked
What does a P0509 code mean?
IAC voltage high.
What causes a P0509 code?
Common causes: Wiring short to power; Failed IAC.
What are the symptoms of a P0509 code?
Typical symptoms: High idle.
Is it safe to drive with a P0509 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $80-$400.
How do I diagnose a P0509 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
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