Mbpro won't boot, fans spin up for a second then nothing

The indicator light will also go on for a split second, and seems to show that the motherboard responds to the different startup combinations (I've tried them all). It still charges the battery.

Previously I had issues with overheating, so I used SMC fancontrol to run fans at full speed most of the time. Then, for a while it would run fans at full speed during boot but never get past a black screen. Now, even less action.

Other things I've tried:

- Disconnecting hardware from the board, such as HD, RAM, screen,

etc.

- try to boot using only battery, only the charger, and both. Even tried disconnecting the onboard DC from motherboard and boot from battery.

- taking everything apart and putting it back together