If you look at the aircraft status screen you should see an ESC item showing there's an error. If you click on the item it will go to a screen that tells you which ESC has the error. The left side board contains ESC 2 and 3, and the right side is ESC 1 and 4, where the motors are numbered counter clockwise starting with the right front. You can fire up the partly disassembled bird without the camera/gimbal installed, but you must be sure to connect all four radio antenna leads or you could damage a transmitter from running it with no load. And obviously don't do that with the props on, since the motors are not fully anchored by screws.