Chosen Solution
OK, so I very carefully installed the new hard drive, a 1 TB hard drive from OWC in the computer, making sure all the cables are hooked up right. And all I get is a fan. Now not even startup manager works, not even zap the PRAM works, not even holding the power button without power connected for 30 seconds work. The same result. Connecting an external hard drive with compatible operating systems does not work via USB. Blank screen, and a fan. Was a connector not installed correctly in the reassembly? Do all the screws have to be inserted in the bezel holding the LCD under the glass, or just enough that it is steady? I hear the computer go bong, wait a few seconds, and then all I hear is the fan. Zapping the PRAM does yield a secondary bong, but not a third one. I replaced an apparently failing 500 GB drive according to DriveDX. Internet restore does not do anything.
The display has a thermal sensor which if not connected will cause the fan to race as the system goes into Safe Mode to protect the system from overheating. As your display was not working I was hoping we could at least see if the logic board was working, hence using an external display to validate that. You could have a damaged cable or the connector on the logic board is damaged. To prevent the magnets from grabbing the screw as you try to put the screws in, you want to do use a piece of tape to hold the screw. I use a piece of electricians tape I punch a small hole in the middle of the strip and press the screw in (from the sticky side) through the tape, then fit the screwdriver into the head of the screw and then fold up the tape onto the shaft of the driver.
