I "upgraded" my desktop HP Pavilion p7-1414 from Windows 8 to 8.1 at the suggestion of the operating system. The upgrade went smoothly and I began using the computer as normal. While I was using it, it powered off without warning and without shutting down. I thought maybe it had overheated for some reason (although it never has before) so I let it sit for a while before trying to turn in back on. When I pressed the power button, the computer began to start up like normal, displaying the HP logo on the monitor. Then it powered off again. Sometimes it will boot up normally, so it isn't the hard drive. But it inevitably powers off again, and often won't stay on long enough to even put anything on the monitor afterwards.
There are no error messages or beep codes. The power supply light on the back of the case is green. I cleaned the inside of the computer case for dust and tested it with a different power cable. Same results. It's plugged into a surge protected strip. Plugging it directly into an outlet doesn't change anything. How long ago I was able to boot it up doesn't seem to have an effect on the likelihood it will boot again. The length of time it will stay powered on seems pretty random.
The problem is intermittent (when I took it to a repair shop it started up pretty as you please and made me look foolish) and I find it suspicious that it began only a few hours after I upgraded to Windows 8.1. But I don't know enough about computer hardware to rule out a hardware issue. Incidentally, this problem started 18 days after the warranty expired. HP Support wasn't inclined to give me grace on the issue. Anybody have thoughts on what might be the cause of this problem?
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Only way to be sure is replace the PSU and test it again.
Powering on and off doesn't sound like an OS issue. It's possible that the PSU was on it's way out, and doing the install drew enough power from the system to finally kill it.
Thank you for the advice.
EDIT: I replaced the power supply but the problem is still going on. I'm sending the system off to a service center and hoping they can figure out what's wrong with it. I'm out of ideas.