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remove any protective case, film, etc > repair the phone with PCC > make sure its a clean update/repair, which means everything on your phone will be deleted.
If that doesn't work, contact your local Xperia care
As suggested ,remove any protective case . It happened with me, I replaced the protective case with one that has a hole where the proximity sensor located and it worked fine. I still use Kit Kat, I'm planing to upgrade to lollipop, but hesitated to do this.