IAC

📖 YOUCANIC Automotive Glossary

Idle Air Control Valve (IAC)

The idle air control valve regulates engine idle speed by controlling a small bypass passage that allows air to enter the engine when the throttle is fully closed. The ECU adjusts the IAC position based on engine load — for example, opening it slightly when the air conditioning compressor engages or when electrical accessories put additional demand on the alternator. Carbon buildup is the most common cause of IAC failure, restricting its movement and preventing it from responding to ECU commands. Symptoms of a faulty IAC include rough or erratic idle, an idle that surges up and down, stalling at stops or when coming off the throttle, hard cold starts, and a check engine light with idle control codes. On many vehicles the IAC can be cleaned in place, while others require removal for proper cleaning or replacement.