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I have been playing around with an old computer of mine, and have found that when I turn on the computer, nothing happens, but if i unplug the 4pin power connector attatched to the motherboard and turn on the power, the fans run, but nothing else does. Any ideas as to what is going on? I remember smelling a slight burning smell, and then the computer powered off.

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Both and report the same symptom pattern: the small 4‑pin CPU power being involved and a faint burning smell before the system died. That connector supplies the CPU power rail (ATX12V), so a short or failure on that rail, a bad PSU, or a failed motherboard power stage (VRM/MOSFET area) are the most likely causes. A failed CPU is possible but far less likely to produce a burning odor by itself.

Try this ordered, safe checklist to narrow it down:

  • Power and safety first: unplug the mains before you touch the board.
  • Visual inspection: look for scorch marks, cracked or bulging capacitors, discolored PCB, blown MOSFETs or chokes around the CPU power area. If you find visible damage, stop — the board is likely fried.
  • Minimal-boot (bench) test: outside the case, with only PSU, CPU, one RAM stick, and onboard video/speaker attached, try powering up and listen for beep codes or watch POST LEDs.
  • PSU check: either swap in a known-good PSU or measure the +12V on the 4‑pin CPU connector (or use a dedicated PSU tester). If the 12V rail collapses only when the connector is plugged to the board, the short is on the motherboard.
  • Look for shorts: loose standoffs, stray screws, or solder bridges can short the CPU power area.

If the board shows physical damage near the CPU power area or the 12V rail shorts with every PSU, the motherboard is the failed component and should be replaced. If unsure or if the parts are valuable, stop testing and get a shop to do a controlled diagnosis to avoid further damage.

Reference material: ATX12V and CPU power role (ATX12V), motherboard power stages (Voltage regulator module), PSU basics (Power supply unit (computer)).

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Member Avatar for raybay

Four pin? Is this a Molex plug, with a square shape?

Member Avatar for mrchill

I have been playing around with an old computer of mine, and have found that when I turn on the computer, nothing happens, but if i unplug the 4pin power connector attatched to the motherboard and turn on the power, the fans run, but nothing else does. Any ideas as to what is going on? I remember smelling a slight burning smell, and then the computer powered off.

I have the same problem with my computer. My PC shutdown unexpectedly, and when I went to turn it on again, it only came on for about 2 seconds. Now there's nothing. The power LED on the mobo is fine, and the fans flinch when I turn the PSU on, but that's all that ever happens. If I unplug the square 4-pin molex from the mobo, I can turn the computer on... that is, the fans and drives run, but nothing else happens. Same thing happens with a different power supply.

I talked to a shop, expecting it was the mobo... but the tech guy there said he thinks it's actually the CPU, since the 4-pin power is for the CPU. So I'm going to see if AMD will return it.

My specs:

Asus A8N-VM CSM mobo
Athlon 64 X2 3800 CPU

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