The imamat of a fasiq is makrooh-e-tahreemi. This means that appointing him as the imam is makrooh-e-tahreemi. If he of his own accord steps forward to lead the salah or someone else appoints him as the imam then the salah performed behind him will not need to be repeated though it will be makrooh. (Kabeeri, pg 351) Hadhrat Anas r.a. and Hadhrat Ibne Umar r.a. would perform salah behind Hajjaj bin Yusuf but there are no reports of them repeating their salah.