Ursprünglicher Beitrag von: Dan ,
Disconnect the keyboard ribbon cable and then use the power pads to start the system are you able to get it to startup that way? If you can carefully reseat the ribbon cable making sure it's squarely and fully inserted. I use a small strip of electicans tape which I fold over one end to make a pull to help in seating the cable. Refer to this IFIXIT guide: [guide|13146|MacBook Pro 13" Retina Display Late 2012 Logic Board Replacement] jump down to Step28.