P0509
Idle Air Control System Circuit High
Same as P0508 with the line shorted to supply or open. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
- Check engine light: Yes
- Emissions related: No
- Repair difficulty: easy
Symptoms
- Check engine light on
- Idle speed too high, too low, or unstable
- Possible stalling at idle or after cold start
Common causes
Ordered by how often each turns out to be the cause.
- Control line shorted to supplyCommon
- Open in IAC valve or harnessPossible
- Connector pins corroded - high contact resistanceLess likely
Parts involved
- Idle Air Control Valve
- IAC Wiring
P0509 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.