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Someone called me saying their computer keeps shutting off. I brought it home to troubleshoot.

It's an HP Pavilion 513c

The computer powers on normally, but almost immediately powers off. Sometimes it manages to make it to a fully loaded desktop, and other times it powers off during the boot process.

The hard drive still spins but there is no video and the CPU fans quit spinning. The power has to be unplugged before you can turn it back on again.

I'm thinking it's either a motherboard or power supply issue. I don't have a spare of either component laying around to test with.

Any guess on which part is more likely to be the culprit?

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The power supply for that unit should have a green light near where you insert the power cord. It should be solid green when powered. – Bon Gart Jul 5 '12 at 20:40
I noticed you said "Nothing with the machine has changed except this problem appeared after a thunderstorm the night before" in your comment to David Schwartz's answer. That is an important piece of information you left out of your question and could very well be the problem itself. – CharlieRB Jul 5 '12 at 20:42

It sounds like the CPU is overheating, the CPU fan isn't spinning, or the CPU fan isn't correctly reporting to the motherboard. Check the heat sink and CPU fan. And make sure the CPU fan is connected to the correct motherboard header.

Also, in the future, it helps a lot to get some problem history. Did they move the computer? Did they clean it? Did they replace any components? Did it fail gradually over time? Or did it suddenly stop working one day? Etcetera.

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The CPU fan spins when powered on but quits when the machine powers off. Nothing with the machine has changed except this problem appeared after a thunderstorm the night before. I'll get into BIOS and check temps tomorrow. Thanks for the reply. – Chase Jul 5 '12 at 2:53

Do a visual inspection of the motherboard. Thunderstorm explains a lot. A Power surge might have been the motherboard's last straw...

Just went through the same thing with an old P4 Dell. The computer kept restarting, never staying on for more than 30 seconds... Then it actually stated that it shut off due to "thermal event". I thought it was overheating but the heatsink never got even slightly warm.

After finally moving some useless plastic shrouding i found a row of capacitors completely burst. Once they go you know its time to shop for a new PC.

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