What must be done when the throttle body is closed and the IAC valve is fully extended to start the vehicle?

When the throttle body is closed and the Idle Air Control (IAC) valve is fully extended, the engine is not getting enough air to sufficiently mix with fuel for combustion. To start the vehicle under these conditions, the throttle body needs to be opened to allow for proper airflow into the engine.

Opening the throttle body fully will create an adequate air passage that allows the necessary amount of air to enter, promoting a good air-fuel mixture and facilitating engine start-up.

In contrast, spraying fuel into the throttle body may help in certain situations to improve fuel delivery, but it does not address the fundamental issue of airflow that is critical for starting the vehicle when the throttle is completely closed. Therefore, the best action is simply to open the throttle body.