Idle Air Control System Circuit High
What does P0509 mean?
The P0509 code means the vehicle's onboard diagnostic system has detected: Idle Air Control System Circuit High. This is a moderate severity code.
Common Symptoms
- Engine idling too high (above 1000 RPM when warm)
- Engine racing at idle
- Idle RPM does not come down after revving
- Poor fuel economy at idle
- Check engine light on
Probable Causes (Ranked by Likelihood)
- Faulty idle air control (IAC) valve stuck open Very High
- Open circuit in IAC valve wiring (high resistance) High
- Corroded or disconnected IAC connector Moderate
- IAC valve mechanically stuck in open position Moderate
- PCM driver circuit issue Low
Step-by-Step Diagnostic Procedure
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Disconnect IAC valve connector -- if idle drops significantly, the valve is stuck open
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Measure IAC coil resistance -- should be 10-14 ohms. Higher resistance indicates an open circuit
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Check IAC wiring for open circuits or high resistance -- measure end to end continuity
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Inspect IAC connector for corrosion or backed-out pins causing poor connection
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Command IAC closed with scan tool -- idle RPM should drop to near stall
Frequently Asked Questions
Can I drive with code P0509?
Yes, driving is generally safe with a high idle, though it wastes fuel and the vehicle may creep forward in drive without pressing the gas. The high idle can also make braking feel different. Get it repaired at your convenience.
What does circuit high mean for the IAC?
Circuit high means the PCM detects higher than expected voltage on the IAC control circuit. This usually indicates an open circuit in the wiring, a disconnected connector, or an IAC valve with an internally open coil, causing the valve to remain in the open or high-airflow position.
Why is my engine idling so high?
A high idle is typically caused by the IAC valve allowing too much air to bypass the throttle plate. When the IAC circuit reads high, the valve defaults to a more open position, allowing excess air in. This raises idle speed above the normal 650-750 RPM range.