You still did not say which revision board you have. The revision number should be located at the bottom left corner of the board.
The following is for a Rev. 1.0 board.
The GA-990FXA-UD3 has 3 revisions: 1.0, 1.1 & 3.0
For the AMD FX-8350:
Rev. 1.0 & 1.1 boards, update to BIOS F9 needed
Rev 3.0 board, BIOS FA (1st BIOS, no updated needed)
Make sure to double check all power connections.
Do you have both 24-pin ATX and 8-pin ATX-12V connectors connected?
Do you have the video card power connector connected?
For example the GeForce GTX 660 requires 1 PCIe 6-pin power connector while the
GeForce GTX 660ti requires 2 PCIe 6-pin power connectors.
Make sure the case front panel is wired to the motherboard Front Panel Header correctly.
All you need is the CPU (and cooler), RAM and video card for a start.
Wired correctly the case power button should cause system to start right up.
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I have a GA-990FXA-UD3 rev. 1.0 board updated with the F9 BIOS.
It has 2x4GB RipjawsX DDR3 1600 MHz. No changes were needed for the RAM voltage.
It booted right up with the AMD FX-8350.