I just got back to KK stock rom few minutes ago, from CM12.1 .  Installed USB Driver from the official LG site,  enter into Download mode and.. the first attempt, after click on Start and OK in the language window with FLash Tools, it doesn't recognize my phone. But I saw that in the end, after all the steps, it open LGMobile Support Tool program, so.. I  closed Flash Tools and opened LGMobile Support Tool, without disconnect the phone... I wen to "Install USB Driver" and wait until the program recognized my phone.. If there is some drivers to install the program will do for sure, in my case, wasn't necessary. After that, went  back to Flash Tools, started all over again.. and Voila!!