Idle Air Control System Malfunction

What it means

The engine computer cannot hold the idle speed at its target, or the idle air control (IAC) valve circuit is not working as it should. On vehicles with an electronic throttle, the throttle body motor itself handles this job.

Related codes: P0506P0507P0171P0121

Symptoms5

• Unstable idle that rises and falls.
• Idle speed too high or too low.
• Engine stalls when stopping or when the A/C switches on.
• Occasional hard starting.
• Check engine light on.

Possible causes6

• Carbon buildup in the IAC valve or its passage in the throttle body.
• Failed IAC valve or its electric motor.
• Open or shorted IAC wiring or a corroded connector.
• Vacuum leak upsetting idle control.
• Dirty or faulty throttle body on vehicles with an electronic throttle.
• Engine computer fault (rare).

How to diagnose5

1. Use a scan tool to watch desired versus actual idle speed and the IAC or throttle opening.
2. Inspect the throttle body and idle air passage for carbon.
3. Command the valve with the scan tool if possible and watch for an engine speed change.
4. Check the connector and wiring, and measure the valve resistance against the manufacturer specification.
5. Look for vacuum leaks by listening or with a smoke test.

How to fix5

1. Clean the throttle body and idle air passage with a dedicated cleaner.
2. Repair damaged wiring or the connector.
3. Replace the leaking hose or gasket.
4. Replace the IAC valve, or the throttle body on vehicles with an electronic throttle.
5. Clear the codes, perform the manufacturer idle relearn, and confirm the idle is stable.

A throttle body cleaning is often enough at low cost; the upper end applies when a complete electronic throttle body must be replaced.

Prices are rough estimates for common cars at independent workshops and vary with the car and the source of the part.
Prices last updated: September 2026

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