There was a time when the Yun was new, that when folks complained about a dead Yun, the first question was whether the white LED was on, but very dim. Whenever the answer was yes, the folks at Arduino recommended returning the Yun for RMA replacement. This was back before the arduino.org folks split off from arduino.cc group. Now that the Yun board is made by arduino.org, and not available from arduino.cc, it would appear that the arduino.cc folks don't pay much (if any) attention this Yun section of the forum?
I've asked, but I never did get any response as to what the dim white LED actually means. Whatever it is, I guess it's fatal?