Below are the steps to start the SharePoint 2013 User Profile Synchronization Service.
Ensure the service account is a member of the local machine Administrators group.
Grant “Replicating Directory Changes” permission
Using the Active Directory Users and Computers manager, grant “Replicating Directory Changes” permission to the service account (SP_Farm).
Restart the SharePoint Timer Service
Start the SharePoint Server Synchronization Service
The service can take several minutes to start.