My system shut down one night. I found out that if I disconnect the 12v connection, the system will boot but I get no video. Even if I re-connect the 12v power cables, I get nothing. I have swapped out the PSU. Do I have a bad CPU or just a bad MB?
Going through EVERY step in the "READ before posting about boot problems!" sticky at the top of the forum should help you find the problem. I can't stress enough that you have to actually go through each step.
I found out that if I disconnect the 12v connection, the system will boot but I get no video.
Boot: This is what happens when an OS successfully establishes itself on your computer. If you have no video output you do not know if it boots... unless the computer happens to talk to you.
POST: Power On Self Test. This is the process the BIOS goes through to verify that your computer is ready to be booted up. You also can have no idea if this is happening or not.
So, what you mean is that you hear some noises when you turn on the machine. That is neither a POST or a boot.
I'm guessing by 12V connection you mean the 8-pin CPU power connection. When you unplug this and turn on the PSU your board gets power, but not enough to actually POST. With this plugged in you get no noise. No spinning fans.