What happens when the idle air control valve goes bad. When the idle air control (IAC) valve goes bad, it can lead to various issues with the engine's idle speed and overall performance. The IAC valve is responsible for regulating the amount of air that bypasses the throttle plate when the throttle is closed. This ensures that the engine receives enough air to maintain a stable idle speed. Here are some common problems that can occur when the IAC valve goes bad: 1.Erratic Idle Speed: The engine