User Profile Synchronization Service failed to start.. can be fixed

if you want to do a User profile import in SharePoint 2010.. you should have the
‘User Profile Synchronization Service’ started. But there are many reason why this
service wont start. Following are some points to check before doing something crazy
like re-install the farm.

Pre-requisites

If you have all of the above set right.. Check the following..
  1. Determine which service account will be used to User Profile Syncronization (UPS) .. Goto CA > Application Management > Manage Services on Server > User Profile Synchronization Service > Click on Start. you will see a defaulting account name already filled in. Make a not of this account. Wait, don’t be in a rush.. just check the account used. When you are finished with all the steps not more than.. come back to this step and enter the credentials to start the service.

  2. Make sure this service account is a domain account with local administrator permissions
    on WFE servers.. if not add the user account to the administrators assemble and Re-boot
    system ( resetting IIS and Timer service may also work.. if not you have to re-boot)
  3. DirSync permissions set for the service account. To be sure give permission for
    both “Replicating Directory Changes” and “Replication synchronization”

    http://blogs.msdn.com/sharepoint/archive/2009/12/14/how-to-set-replication-directory-changes.aspx

  4. Service account has full control permissions on User Profile Service attention. for this goto Central Admin >Attention Management >Manage Service Applications >Click on the bar of “User Profile Service Attention ” (.. dont click on the link). In the ribbon, make sure that under Administrators and Permissions Menu, the service account has full control.
  5. Make sure both Forefront Identity Administrator services are configured to run with the
    service account identified. you can configure logon account by rightclick on service
    > properties > log on .
    Note: that the services may not be running. it will involuntarily
    start when the User Profile Sync starts.

  6. Go back to step 1 and fill in the credentials and start service
Once all the above is done.. you can start the service.. steps provided here
Configuring User Profile Service Attention in SharePoint Server 2010
Exceptions:

Logs force show you the following error “Exception trying to write the dbName regkey for MIIS System.Security.SecurityException: Requested registry access is not allowed”.
This is because the service account used to start the user profile sync is not the administrator of the local system and may not have rights to modify the registry.
Solution is to add the user account to the administrators assemble and Re-boot
system.

UserProfileServiceUserStatisticsWebPart:LoadControl failed
UserProfileServiceAudienceStatisticsWebPart:LoadControl failed
UserProfileServiceImportStatisticsWebPart:LoadControl failed

Exception: System.IO.FileLoadException: The located assembly’s manifest classification does not match the assembly reference. (Exception from HRESULT: 0×80131040) at Microsoft.Office.Server.UserProfiles.UserProfileConfigManager.InitializeIlmClient

Resolution : just do a IISReset.

Check it out:SharePoint 2010