P0509 means Idle Air Control System Circuit High. The idle air control valve circuit voltage is higher than expected, often from an open circuit. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0509 means
Most vehicles built in the last two decades use electronic throttle control instead of a throttle cable: the accelerator pedal carries two or three position sensors, and the throttle body has a motor and two position sensors of its own. The computer compares the redundant sensors constantly, and any disagreement triggers reduced-power mode as a safety measure. Older engines with a throttle cable control idle with a separate idle air control valve.
P0509 symptoms
- High or unstable idle
- Stalling
- Check engine light
Common causes of P0509
- Unplugged or corroded connector
- Open wiring
- Failed idle air control valve
How to diagnose P0509
- Check the connector
- Check the circuit for opens
- Measure valve resistance
Can you drive with P0509?
Usually yes.
P0509 tips
Check that the idle air control valve connector is fully latched after any throttle body work. If the connector is good, check each circuit back to the engine computer for an open; corroded terminals are common on older vehicles, and some valves need an idle relearn after replacement.
Will P0509 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0509 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0505 | Idle Air Control System Malfunction |
| P0507 | Idle Air Control System RPM Higher Than Expected |
| P0508 | Idle Air Control System Circuit Low |
Guides that help
When it is more than a sensor
Throttle and idle codes are fixed at the throttle body, the pedal, a sensor or a leak. If those check out and the engine still runs poorly, a compression test separates a control problem from a worn engine; compare engines by code on our engines page.
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