P0518 moderate

Idle Air Control Circuit Intermittent

The P0518 code means the vehicle's onboard diagnostic system has detected: Idle Air Control Circuit Intermittent. This is a moderate severity code.

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Keep driving?: Short distances only
DIY difficulty: moderate
Estimated cost: $50 - $300 (parts and labor)

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Common Symptoms

Check Engine Light on
Idle speed fluctuates randomly
Occasional stalling at idle
Idle hunting up and down

Probable Causes (Ranked by Likelihood)

Intermittent connection at IAC valve connector Very High
IAC valve sticking from carbon deposits High
Wiring fault causing intermittent signal loss to IAC Moderate
Failing IAC valve motor with worn contacts Moderate

Step-by-Step Diagnostic Procedure

Wiggle the IAC valve connector while monitoring idle speed for changes
Remove and inspect the IAC valve for carbon buildup causing intermittent sticking
Check wiring from ECM to IAC valve for intermittent opens, especially at flex points
Monitor IAC commanded position vs actual idle RPM over time; erratic behavior confirms intermittent fault

Frequently Asked Questions

Why does my idle speed go up and down?

An intermittent IAC valve circuit causes the ECM to lose control of idle airflow briefly. When the signal drops out, idle speed changes. When it reconnects, the ECM corrects, creating a hunting or surging idle pattern.

Can I clean the IAC valve to fix this?

Cleaning the IAC valve is a good first step, especially if carbon buildup is causing the valve to stick intermittently. Use throttle body cleaner and a soft brush. If cleaning does not resolve it, check the wiring and connector.

Is P0518 different from P0511?

Yes. P0511 indicates a continuous fault in the IAC circuit, while P0518 indicates the fault is intermittent (comes and goes). Intermittent faults are often caused by connector or wiring issues rather than a completely failed component.