After he became a teenager, he served in the palace guard corps, and when Emperor Wen saw him, he was reminded of Yuchi Chong's contributions.
He therefore issued an edict giving Yuchi Yichen a moderate amount of wealth, while bestowing the imperial clan surname of Yang on him and conferring him the status of the emperor's grandnephew.
Soon afterwards, Emperor Wen made him the governor of Shan Province (陝州, roughly modern Sanmenxia, Henan).
Yang Yichen was considered to be honest and careful, with military leadership qualities, and Emperor Wen respected him.
In 600, in conjunction with Shi Wansui (史萬歲) the Duke of Taiping, he dealt Ashina Dianjue a major defeat.
Sometime during Emperor Wen's late reign -- the Renshou era (601–604), Yang Yichen was promoted to be the commandant at Shuo Province (朔州, roughly modern Shuozhou, Shanxi).
In 604, Emperor Wen died—a death that traditional historians, while admitting a lack of direct evidence, generally believe to be a murder ordered by his son, Yang Guang the Crown Prince.
Yang Yichen then engaged another major rebel leader, Gao Shida (高士達), who had claimed the title of Duke of Donghai.