Why does my PC successfully boot only when unplugged for more than a few minutes?

I have an HP Pavilion Elite desktop computer, model HPE-490t. I like it because it didn’t cost too much, boots itself from an SSD, came with 16 GB of RAM, and has six CPU cores for editing video and camera RAW images. It has one behavioral quirk that I cannot explain, however. The recent power interruptions here in the Northeastern United States got the machine into a state where it could not be restarted. It would power up for a second or two, shut down, and then power up again, never being able to get to the point of showing anything on the monitor.

• I unplugged it for about 10 seconds and plugged it back in. Same behavior (fails to boot).

• I unplugged it and walked away for an hour, then plugged it back in and it worked perfectly!

I think something similar happened after installing a second hard disk drive into this machine.

Why does the computer behave differently depending on how long it has been unplugged? Where is energy stored that affects the machine’s ability to boot? Capacitors in the power supply? Battery on the motherboard (there is one for the clock, but that wouldn’t be exhausted by being unplugged for an hour, I don’t think)?

If it's consistent and leaving it unplugged fixes it, it sounds like it could be overcharged capacitors.

Try removing the power entirely, then holding the power button down for a minute or so. This should COMPLETELY drain everything off, and hopefully it'll continue working fine afterwards. If your power source is pretty dirty, you might need a decent active UPS though. This procedure is NOT something you should have to be doing regularly.

It sounds to me like some sort of hardware failure. It could be on the mainboard, the power supply, CPU, or any number of other places. But I think it is most likely to be on the mainboard or power supply.

Power interruptions won't generally cause problems with CPU overheating. It's possible, but not very probable.

If the unit is still under warranty you should contact the store you purchased it from or HP first.

If it is not under warranty, power supplies are relatively cheap and relatively easy to replace, so try replacing that first. Mainboards from OEM manufacturers (HP, Dell, other big-name brands that will customize the hardware they use for various reasons) can be very, very expensive, usually half or more the cost of the entire system. If replacing the power supply does not resolve the issue and the unit is not under warranty, I'd check the costs and see if it might not be a better value to replace the entire computer.

My rule of thumb regarding the repair/replace value is between 50% and 75%: Unless the system is very valuable or otherwise irreplaceable, if the cost of repair is more than 50% to 75% the cost of a comparable new computer, it will be a better value simply to replace the entire computer. Keep the hard drive and buy an external tray that it can plug into. That way you'll have a backup device and all your data.

If you go about trying to replace the mainboard using off the shelf components, keep in mind you'll probably need to fiddle around with drivers and may be better of reinstalling the entire operating system.

And, most of all, to minimize the chances of this happening again: Buy an uninterruptible power supply (UPS). It's a battery backup for your computer that does more than just smooth out spikes. The good ones will condition the power, allowing maximum life from your components, especially in areas with frequent power fluctuations.

I had almost the same (visually) effect with my wife's host some time ago. I had to replace the power supply (as lazy person) due to, as Shinrai mentioned, capacitor problems.

About the time-slot difference: capacitors should get rid of the residual charge, for "bad" capacitors it lasts longer. An alternative solution for a long full power-off may be to "press the power button long time on power-on". At least "some minutes" helped me 50/50.

If anybody is still facing such problem, it is because of burst capacitors in your motherboard or in the power supply unit. If possible, disassemble the unit and check; you can compare the bad capacitors easily. They are burst or swollen upside usually.

If you can solder yourself you can replace those capacitors with the same value: voltage and microfarad capacitors. If you are not confident about it, go to a maintenance shop.