Why does engine idle down and shutts off when put into gear?

An engine may idle down and shut off when put into gear due to several potential issues. Common causes include a malfunctioning idle air control valve, which regulates airflow and affects engine speed, or a problem with the throttle body. Additionally, issues with the transmission, such as low fluid levels or internal damage, can cause the engine to stall when engaged. Lastly, a faulty sensor or vacuum leak might disrupt the air-fuel mixture, leading to stalling when the load increases upon shifting into gear.