Question PC wont turn on, temporarily turns on when CMOS is reset while computer is on

May 4, 2019
Hey everyone,
I've been having an issue with my computer that I've been trying to fix for the past few weeks but cant definitively get to the cause of the problem, and wanted to get some extra opinions.

It started when a few months ago my friend accidentally plugged in a USB into a broken port. The computer immediately shut off as soon as the USB made contact with the port, and has since not turned on. Now, being that this is an old desktop that I don't use anymore (I only use my laptop nowadays since its much better) I didn't really care too much to fix it. However, a few weeks ago I decided I'd try my hand at trying to fix the issue, mainly as a fun learning experience.

When I press the power button, the lights on the motherboard and some components will turn on for a split second, but the computer will never actually start. Every time after this that I press the power button, the components just wont light up at all, until I flip the power switch off and then turn it back on, in which case it'll do the same thing - flashing for a millisecond and ultimately not turn on at all. It will not POST, and no beeps will come from the motherboard (It has a built in LED display for POST codes and a speaker attached to the motherboard). It looks totally dead. So, I began the troubleshooting process. It was fairly easy to eliminate most of the components as the cause of the issue, and eventually I was left with three possibilities - the power supply, the motherboard, or the CPU. I was able to test my power supply with a power supply tester, and found that it was working perfectly fine. That left it down to either the motherboard or CPU. Here's where it got tricky. Since I don't actually have any spare components, it's been really hard to definitively say which one is at fault, though at this point I'm fairly sure its the motherboard.

At this point I took my motherboard out and started trying a whole bunch of troubleshooting on a wood plank after ripping apart the entire computer. That alone eliminated any possibility of it being a short caused by any loose metal. I tried removing and placing back in the CMOS battery, clearing the CMOS using every method possible while the computer was OFF, reseating the CPU, booting the computer without the CPU (I only had the power supply and motherboard connected and that was it), and other things, and each and every time, no matter what I did, the same problem occurred, and it behaved the same way, flashing for a moment, but never actually turning on (Never POST, never any beeps) with or without the CPU.

However, the ONLY thing that seems to have changed the behavior of the computer at all has been, (forgive me for my stupidity) clearing the CMOS while the power switch is set to on and power is being supplied to the motherboard. Now, I know that this is extremely stupid, and that you should always clear the CMOS while the computer is off, but its an old computer that I don't use, so I thought I'd try it. If I clear the CMOS after attempting to turn the computer on, (Either by using a screwdriver on the jumper pins, or pressing the button to clear the CMOS) then the computer will actually start to run. All of the lights will turn on, all the fans will run (Except for the CPU fan, oddly) and everything internally will look okay (Minus the fact that it still wont POST, and the POST code LED's dont turn on, and it still doesn't beep). However, it wont actually give any display, which leads me to believe it isn't actually booting, especially since it doesn't appear to POST. Now, this is the only scenario where the computer wont completely refuse to turn on, but even so, this is only a TEMPORARY fix. As soon as I turn the computer off again, it will repeat the same exact broken behavior. Each time, clearing the CMOS while the power is on seems to make the fans and lights function as I described above.

All of this has led me to believe the motherboard is fault, and that I'll just need to buy a new one. Still though, I'd love to hear what you guys think about this, because as I said, this is all for the purpose of fun learning for me, and I'd love to learn as much as I can, so if any of you have any expertise on the issue, it would be greatly appreciated! I've spent the past couple of weeks scouring the internet and I can't find anything that sheds any light on this specific issue, or that has helped me fix it in any way, so finally I turned here. Since I don't have a spare computer to test my CPU on, though very very unlikely, does anyone believe theres any chance it COULD be the CPU, and not definitively the motherboard? (Excuse my noobish question). Or is it pretty safe to go ahead and just replace the motherboard? Also - is there any possible way I might be able to repair the motherboard, and if so, what kind of tools would I need to do so? (Multimeter, etc.). I know this has been a long post, so if you've made it this far, I whole-heartedly appreciate you bearing with me, and would love any input you all have!

P.S. - Something worth mentioning, this issue has actually happened in the past, caused the exact same way, when a USB was plugged into the same faulty port. It was years ago, and I had a friend fix it, so I have no idea how they did it, (We think maybe they bent a pin into place that was causing a short - possibly?). Even so though, they fixed it fairly easily I remember, so I'm guessing this time the motherboard just got totally fried, and last time it was a less severe, easily fixable issue.

Thanks everyone in advanced for your feedback!

Motherboard: Gigabyte GA-Z77X-UD3H