Hi, Try starting it outside the case (on the motherboard's box). See if it does work.
I gave that a try and the same backwards 6 flashed for less than a second and nothing happened. I forgot to add that when I hit the power button and it flashes the code, nothing else inside the case turns on or tries to turn on for that matter. My PSU is a EVGA Supernova 750G also by the way