Computer turns off after 2-3 seconds

My computer has been running fine for many months (or even years? I can't recall for sure). Today, when I tried to turn it on, it began starting up for about 2-3 seconds, then just turned off. The motherboard light is on the entire time. I tried unplugging everything outside the computer (monitor cables, USB stuff, etc.) but that did not help. I opened it up, cleaned it out with compressed air, and that did not help.

I also noticed that after one of these failed startups, subsequent presses of the power button do nothing. The only way to get it to do the "2-3 seconds of startup and then turn off" thing is to unplug it for long enough that the mobo light turns off, and then plug it back in.

I tried removing the graphics card. After this, the computer did boot up. However, I have no integrated graphics, so I had no way to plug in a monitor and see what was going on. It stayed running for about a minute or so, and I turned it back off and replaced the graphics card. This time it booted up past the 2-3 second spot, and stayed working for around 30 seconds, so I turned off the computer myself again. I hooked everything back up, and tried booting. It shut off after 2-3 seconds.

The monitor(s) never come on, so I cannot see anything there, and it only stays on for 2-3 seconds anyway. So I certainly cannot look in BIOS or anything. It seems obvious that it is a hardware problem, but I have no idea which piece. The graphics card seems like the most likely culprit so far.

Any ideas?

 

Ok, I've done some more tests. My wife's computer has the exact same video card, so I switched them. My computer still has the same problem, but hers seems to be running fine with my video card.

This is really confusing, as unplugging the video card from my computer causes it to stay on, but it is apparently not the card. I should also add that when I first switched the cards, and my computer was still laying open, it actually started up all the way to windows just fine. As soon as I stood it up and put it back together, it was right back to 2-3 seconds and then turning off.

 

My guess is a bad power supply.

Even a bad PSU usually puts out one or two watts - enough to power the pilot light on the board. It's similar to a bad car battery: the dome light will come on; however the starter won't turn over.

Try swapping the PSU from your wife's PC (or, if the PSU is an OEM cheapie that's over a few years old, save yourself some time and just buy a new one for your PC ).

Hope this helps.

 

After further consideration/tests, it indeed looked more and more like a PSU thing. I actually managed to find a similar (slightly better) Corsair brand one at Best Buy, so I already bought and installed it. So far, everything is running ok. It went to BIOS and booted into Windows as if nothing ever happened

I am on it right now writing this, so here's hoping everything is ok. I'm still terrified it will just shut off at any moment, but things are looking up so far. Any idea how scared I should be?

 

I'm thinking your heat sink paste CPU to the heat sink failed. Why the computer shut down after a few minutes. How did the paste look when you swapped them out?