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My homebuilt PC died last night after running fine for several months in its current configuration.

My external USB HD (MyBook) was acting up so I unplugged it and rebooted. When Windows started again I plugged the drive in, it initialized, and then the PC rebooted. It didn't POST however and the case HD light was steady on. I tried resetting via the case, same result. I unplugged the power, waited, plugged back in and got nothing at all when I tried the case switch. The motherboard status light is lit, however.

I've since removed all cards, memory, etc. and reseated everything, still nothing. I've also removed the CMOS battery, waited, reinserted, still nothing. I've replaced the CMOS battery with a fresh one, still nothing.

I'm suspecting the power supply (I'm about to try it in a different machine), but could it be anything else?

Specs:

Mobo: ASUS P5L-MX
CPU: Intel C2D E6300
RAM: Corsair XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 675 (PC2 5400)
PSU: Coolmax AP-550X (CP-500B)

1 SATA HD, 1 IDE HD, and 1 IDE DVD-RW also typically connected

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Dani AI

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Both and describe the same core symptom: the motherboard shows standby power (the lone LED) but the system never spins fans or goes through POST. That pattern usually means the 5VSB rail is present but the board is never telling the PSU to switch on (or the PSU fails under load). The ATX signalling basics (PS_ON, PWR_OK, 5VSB) explain this behavior: https://en.wikipedia.org/wiki/ATX

Try these focused checks before condemning the board or swapping more parts:

  • Bypass the case switch. Momentarily short the motherboard power-header pins with a nonconductive-handled screwdriver (one quick touch). If it starts, the case front-panel or its wiring is suspect. Take care to avoid shorting neighboring pins.

  • Bench the board out of the case on a nonconductive surface. That rules out a standoff or screw short. Connect only CPU, cooler, one RAM stick (if required for fan spin), PSU 24-pin and CPU 4/8-pin and try powering it.

  • Inspect connectors and solder joints. Look for cracked/bent pins at the 24-pin and CPU power plugs, loose latch clips, frayed wires, or poor crimping. Wiggle connectors gently while trying to start to reveal intermittent contacts.

  • Look for visible failures: blown or bulging capacitors, burn marks, or a burnt-odour. These point to component-level board failure.

  • Measure rails and control signals with a multimeter or PSU tester. 3.3V, 5V and 12V should be within about ±5% under load; the 5VSB should be ~5V. Verify whether PS_ON is being pulled low when the button is pressed—if it is not, the motherboard likely isn’t requesting start.

If the board never asserts PS_ON but the PSU tests good under load, the motherboard is the main suspect. If voltages collapse only under load, the PSU is flaky even if it once ran in another system. For clear procedures on PSU testing and rail expectations see a PSU primer:

If these steps are uncomfortable or inconclusive, consider a local repair shop or RMA—intermittent faults (marginal solder joints or failing components) often need bench tools to isolate.

Member Avatar for SiW

Update:

I tried the PSU in another machine (a Pentium 4-based system) and it worked fine. Unfortunately I don't have another compatible PSU to try in my dead machine to officially rule out the PSU, but seeing as the existing one works fine in the other machine it does sound like it could be something else - motherboard?

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Update 2:

I got hold of another comparable system (as far as PSU goes) and tried its PSU in my machine - fans started up. So it sounds like it was the PSU, yeah?

Then I tried my PSU in the other machine - fans started up fine there too. So I don't know what the problem is!

Member Avatar for kostyak

I wanted to find out if you ever resolved this problem. I believe I am having a very similar issue -- I've had my PC for 3 years, and until today it's been working just fine. But, when I tried to boot it up today it had absolutely NO signs of life (no fans spinning, no front panel lights on, etc) except for the lonely green LED light on the motherboard. I tried disconnecting all of the peripherals, disconnecting and reconnecting all cables from the power supply, replacing my RAM, but still the same result.

Does anyone have any ideas what I should try next? I have:
* Asus K8V SE Deluxe
* AMD Athlon 64 3400+
* 2 x Corsair XMS3200/1024MB/400Mhz
* ULTRA X-Connect® 500W ATX PSU

Thanks,
Kostya K.

Member Avatar for SiW

I can only suggest trying a different power supply, as I was never able to find any other reason.

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