DSi blue light flashes and nothing else happens

I have a dsi that the blue light will flash when I press the power button. The top and bottom screen kinda turn a different shade of black for a split second, but nothing else happens. I have tried:

new battery

new daughter board

replaced both screens

cleaned all cable connectors

checked for water damage

soft reset/ hard reset in case of missed update

Replaced the wifi board

I even removed the motherboard and placed it into a known working DSi, but it still had the same issue. The mother DSi is my brother's and has been very well kept. It has no cosmetic or physical damage at all. What would I be looking at as possible reasons it doesn't work?