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I have an Asus Sabertooth 990FX (revision 1, I think) with a FX-4100 CPU. I decided to go ahead and upgrade to a FX-8350.
First, I put the new CPU in and found that it wouldn't pass POST. So, I read the ASUS website and discovered that I needed to update the BIOS to support the new CPU. After swapping the CPU to the old one, upgrading the BIOS and swapping back to the new CPU, it still doesn't POST. The LEDs on the motherboard blink normally - CPU, then RAM, then CPU again - but nothing stays on to indicate a bad component and the troubleshooting LEDs don't proceed to the video card or BIOS LEDs. I don't have a PC speaker to test, but if the CPU was bad, the manual says the LED next to it should stay on.
I also tried with only 1 stick of memory and default BIOS settings.
Is my new CPU bad, or is my motherboard not really compatible even though ASUS claims it is?