It is not permissible to pray behind an imaam who is a charlatan and liar
Question

                Is it permissible to pray behind an imaam who is a charlatan and liar, knowing that some of these people can recite Qur’aan very well? Please advise us, may Allaah reward you with good.
Praise be to Allah.If the imaam is a charlatan who claims to have knowledge of the unseen 
          or who does false and evil actions, then it is not permissible to take 
          him as an imaam or to pray behind him, because whoever claims to have 
          knowledge of the unseen is a kaafir – we ask Allaah to keep us safe 
          and sound. Allaah says (interpretation of the meaning):   
“Say: “None in the heavens and 
          the earth knows the Ghayb (Unseen) except Allaah” [al-Naml 
          27:65]   
The same applies to one who practices sihr (witchcraft or magic) – 
          he comes under the same rulings as the kaafir, because Allaah says (interpretation 
          of the meaning):   
“They followed 
          what the Shayaateen (devils) gave out (falsely of the magic) in the 
          lifetime of Sulaymaan (Solomon). Sulaymaan did not disbelieve, but the 
          Shayaateen (devils) disbelieved, teaching men magic and such things 
          that came down at Babylon to the two angels, Haaroot and Maaroot, but 
          neither of these two (angels) taught anyone (such things) till they 
          had said, ‘We are for trial, so disbelieve not (by learning this magic 
          from us)’…” 
[al-Baqarah 2:102]   
          
But if he is a person who commits some sins, but he does not do any 
          actions of kufr such as practising witchcraft and claiming to have knowledge 
          of the unseen, then prayers offered behind him are valid, but it is 
          better to find someone else who is of sound character and is righteous, 
          to be on the safe side and to avoid differing with those scholars who 
          say that it is not permissible to pray behind (such a person).  
          
Sinners should not be taken as imaams, but if they are already in 
          that position, prayers offered behind them are valid, because it may 
          be that the people are being tested by means of them, or necessity may 
          force us to pray behind them. But in the case of one who calls upon 
          anyone other than Allaah, or who seeks the help of the dead, we should 
          not pray behind him, because this action makes him one of the kuffaar, 
          for this is the action of the mushrikeen whom the Prophet  
          (peace and blessings of Allaah be upon him) fought against in Makkah 
          and elsewhere. We ask Allaah to reform the Muslims and to grant them 
          understanding of His religion, and to cause the best of them to become 
          their leaders, for He is the All-Hearing, Ever ready to respond.