Member Avatar for pluskin

I had been having a problem for a while with my CPU overheating. Having built the computer maybe 3 years ago, I thought it might be due to have the thermal paste reapplied. So I got some thermal paste and thermal paste remover, reapplied to my CPU and heat sink. When I plugged the computer back in, and switched the power on, the only response I got was really strange. The computer would not power up. But the fans would all pulse over about a 3 second cycle, along with the LED lights.
I decided that I must have done something to screw up the CPU. Its an AMD Athlon and I know they are delicate, so maybe I accidentally damaged it while installing the heat sink, or something else along those lines. I procured another CPU off from eBay. Installed it. Exact same problem.
So previously I was sure I had a CPU problem. But now... could I have really screwed up the CPU installation twice? Could I have bought a bad CPU? Maybe I did something to the mobo without realizing? Its a strange symptom so I don't know what to make of it.

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Short-cycling (fans and LEDs powering for a few seconds, shutting off, repeating) is almost always a power or protection problem — the board or PSU is trying to start then immediately cutting out. In this thread ultimately found a loose motherboard power plug, which is a very common and easy-to-miss cause. The other replies from , and point toward the usual suspects (mobo, PSU, RAM), so here is a short, practical checklist in the order that saves time and expense.

  • Unplug power and verify the basics: wall switch, PSU switch and the mains lead. A fully-seated mains and ATX cable are worth checking first because they are the simplest fixes.
  • Inspect power connectors and CPU area: make sure the 24-pin ATX and the CPU 4/8-pin lead are fully latched; look for melted plastic, broken latches or bent pins (AMD CPUs use pins on the CPU).
  • Do a minimal “bench” boot (outside the case): motherboard, CPU+cooler, one stick of RAM, PSU and video (if needed). This removes case shorts and bad peripherals from the equation.
  • Memory and front-panel: reseat DIMMs and try one module in the recommended slot; disconnect the case power switch (or momentarily short the two power pins per the manual) to exclude a faulty front-panel switch.
  • Check the PSU properly: swap in a known-good PSU or use a PSU tester/multimeter to confirm the +12V/+5V/+3.3V rails stay within tolerance under load.
  • CMOS/beep codes: clear CMOS per the motherboard manual and attach a system speaker or use onboard debug LEDs to read POST codes.

If the bench test boots, re-examine case standoffs, front-panel wiring and connectors. If it still cycles, the fault is very likely PSU or motherboard — swap one at a time and bench-test each replacement. Always work unplugged, take ESD precautions, and avoid forcing connectors; loose connectors are embarrassingly common but fix quickly.

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Member Avatar for tglems

I'm having a similar problem of turn on for 3 secs/off for 3 secs/on for 3 secs, etc.
I checked my PSU and RAM and HDD and basically everything except the CPU and mobo, and everything else worked fine. I then replaced the mobo and it did the turn on/off thing once and then proceeded to boot right up. Strange that every time I turn it on it will do the on/off cycle, and I'm still trying to get answers, so if you have found any please let me know, but in the meantime I would suggest that you replace the mobo and see if that fixes your problem. It's possible that you damaged it when working replacing the CPU's. Before you do that you should of course reset the CMOS (just remove the battery for a couple mins) although I doubt that will work for you as it did nothing for me.
Best of luck!

Member Avatar for fren

Pull the memory sticks out and put them in again. Clean their gold contacts with a pencil eraser. Test it one a time in different slots.

Reset your cmos memory, refer to your motherboard manual.

Check all the power connections from the power supply to the motherboard. Test your system with a known good power supply.

Hope this helps...

Member Avatar for cguan_77

ho, check your psu it could be a dying psu..

Member Avatar for pluskin

Any easy way to check the psu?

Member Avatar for cguan_77

hi pluskin, short the green wire to any black wire, this will power on your PSU..and check the voltages... the red wire should be +5 V, orange +3.3 V, Yellow +12 V, check the web for the complete output voltage of the ATX power supply

Member Avatar for pluskin

I finally found the problem. I feel pretty stupid, but all this time, the power supply plug to the motherboard was loose. I must have partially pulled it out while working on the CPU. I was planning on testing the PSU, and when I saw how easily it pulled out I figured I knew what the problem was.

Thanks for the suggestions.

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