Li Chengqian
Du made several suggestions to Li Chengqian, asking him to change his ways, but Li Chengqian did not do so. Du told Li Chengqian what Emperor Taizong told him—which led to Li Chengqian immediately submitting an objection to Emperor Taizong. Emperor Taizong asked Du, "Why did you let him know what I said?" Du responded, "He did not listen to me, so I wanted to scare him with your words. I hoped that he would be scared and would change his ways." Emperor Taizong was still displeased, and he demoted Du to the post of prefect of Gu Prefecture (穀州, in modern Luoyang, Henan), and later to be the commandant at Jiao Prefecture (交州, roughly modern Hanoi, Vietnam). After Li Chengqian was revealed to have plotted with the general Hou Junji to overthrow Emperor Taizong in 643, Du was accused of having received a gold belt from Hou, and he was stripped of his rank and exiled to Huan Prefecture (驩州, also in modern Vietnam). However, later he successively served as the prefect of Ying (郢州, roughly modern Wuhan, Hubei) and Shi (石州, roughly modern Lüliang, Shanxi) Prefectures.