The following is from the second post in this forum, which, in my opinion, does a terrific job of describing the process of troubleshooting a motherboard that won’t POST.
Disconnect the power supply from the motherboard. All power supply wires disconnected from any load. Plug in the power supply and switch it on at the back. The power supply fan should not start spinning on its own. For the power supply fan to start spinning, the PSON# signal on the main connector has to be grounded. If the power supply fan starts to spin on its own, then the power supply is not meeting spec. With the main power connector on the power supply not connected to anything, there should be no way to get the fan inside the power supply to spin.
For the next test, start with the blank motherboard, with no components plugged into it. Plug in the main ATX power connector and the 2x2 ATX12V power connector. Turn on the power supply at the back. Does the power supply fan start to spin immediately, with no other components on the board ? If it does, return the motherboard to your vendor, as it is bad.
When doing this test, I sit the motherboard on top of a cardboard covered phone book. That avoids any (slightly conductive) antistatic materials, such as the motherboard bag or the foam used in some motherboard boxes, from upsetting any sensitive quartz oscillator circuits on the board. There are a few nodes on chips, that can be sensitive to externally applied DC bias, so weighing the risks, I use a cardboard covered phone book as my base. Use antistatic precautions before handing the board, draining any static charge off your hands before touching the board. The case of the PSU is handy for this.
Next, insert the processor+heatsink+fan. Connect the computer speaker to the PANEL header. Connect the front panel power switch to the PANEL header. Turn on the power supply. Does the power supply fan start to spin right away ? If it doesn’t, now press the equivalent of your front panel power switch. Does the computer speaker beep the pattern for “Missing or bad RAM” ? If you get a beep pattern, that means the CPU is executing BIOS code properly.
Add the RAM but no video card. You should hear a beep pattern for “No video”. If adding the RAM kills the board, then the RAM is shorting something out, and is bad.
You can incrementally test the responses of the motherboard, while it is sitting outside the computer case, and the results of each test can help you decide what component is causing grief. For my last two builds, I completely constructed the system on a table top, including booting into Windows. And then I could safely waste the time putting all the components into the computer case, knowing they are all working.
I had one motherboard here, where an IDE cable got pulled part way out of the connector. Half the pins were making contact and half were not. That caused my computer to start immediately when the power was switched on at the back of the computer. And what that means, is when the Southbridge is stressed by an electrical disturbance, it is possible for the power switch logic not to work properly. So if any interface on the Southbridge is being stressed, you could get those symptoms.