Idle Air Control System Performance

Possible causes

• CommonIdle air control valve clogged with carbon
• CommonFailed idle air control valve or motor
• CommonVacuum leak in the intake system
• CommonDirty throttle body
• PossibleCorroded connector or damaged wiring
• RareFailed control module driver

What you might notice

• Idle too high, too low, or hunting up and down
• Stalling when coming to a stop
• Stalling when the air conditioning switches on
• Hard starting
• Check engine light on

What to check, in order

1. Look for vacuum leaks before anything else - a split hose, a failed PCV, a leaking intake gasket. A performance complaint with a healthy circuit is usually unmetered air.
2. Compare commanded against actual idle speed on the live data screen.
3. Check for vacuum leaks first. A leak lets in air the module did not account for, and it produces every idle complaint that a failed valve does - at a fraction of the cost.
4. Remove and inspect the idle air control valve for carbon. On a high-mileage engine this is the usual cause and cleaning is often the whole repair.
5. Clean the throttle body while you are there.
6. Check the connector and wiring at the valve.
7. Check whether an idle relearn is specified after cleaning. Some modules keep using the values they learned around the dirty valve until they are reset.