A stuck closed IAC (Idle Air Control) will result in what condition?

A stuck closed Idle Air Control (IAC) valve primarily affects the engine's air intake, which can severely disrupt the idle air mixture. When the IAC valve is not allowing air to enter the engine as needed, it leads to an insufficient amount of air during the idle phase, causing the engine to try to compensate by operating with a very lean fuel mixture.

This situation can prevent the engine from starting properly, as the lack of airflow makes it difficult for the engine to maintain combustion at idle speeds. Once the engine does start, the inadequate air supply may not sustain engine operation, which can lead to the engine starting and then dying shortly thereafter. This is why the condition associated with a stuck closed IAC valve is accurately described as "start then dies," since the engine may momentarily start but cannot keep running due to inadequate airflow.

While a stuck closed IAC might also cause a rough idle or hard starting in different contexts, the most direct and immediate effect linked to the stuck closed position is the engine's inability to run properly after starting, leading to the "start then dies" condition. Thus, this makes the best choice reflect the primary and critical consequence of the IAC's malfunction.