sparki wrote on 2025-05-11, 12:04:

Hello! Recently i've acquired an asus p4p800 and when i tried turning it on, it didn't show any signs of life except the power l […]
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Hello!
Recently i've acquired an asus p4p800 and when i tried turning it on, it didn't show any signs of life except the power led on the board. I suspected there was a short and i've discovered there is a short in the 4 pin cpu power connector and the board powers on if that connector is unplugged.

I kinda don't know where to go from here. Does this mean the power mosfets are shorted and need replacing? I remember reading someplace this is not very uncommon for these boards. Is it possible to measure the mosfets with just a multimeter?

Thanks in advance

Did you see any signs of burn around the 4-pin CPU power connector? I'm afraid boards of that period may be prone to having that connector eventually burn out after prolonged use, which will result in a short. Perhaps the connectors used on those boards were of bad quality that could lead to higher-than-usual resistance around the connection.

I've a 865-based board (IMB200) that failed in a similar manner. After years of prolonged use the board's 4-pin CPU connector started showing signs of burn although it could still work. Eventually the board shut itself down and I could not power it on anymore, and can only be powered on if the 4-pin connector is unplugged -- of course it won't boot. Haven't dug deeper into the issue as I replaced the board afterwards. After removing the board from the case I checked the 4-pin CPU connector with a multimeter and it was indeed shorted.

You may consider desoldering the 4-pin CPU power connector and see if the short is still present on the pads on the motherboard. If not, it's possible that the board may become usable again after soldering a new 4-pin CPU power connector.

Desoldering components from large PCBs such as motherboards may not be an easy task without the right tools, however...

It could also be a shorted FET or capacitor in the VRM causing this. With CPU removed I would check the FETs....

Horun wrote on 2025-05-11, 18:26:

With CPU removed I would check the FETs....

^ This.

Short between all 4 pins on the 4-pin 12V CPU connector = short-circuit or overload somewhere on the CPU VRM.

Step 1) Remove CPU and see if the short-circuit is gone.
- If YES, you more than likely have a shorted "upper" MOSFET.
- If NO, you have some other component shorted between ground and 12V - either a ceramic cap, electrolytic cap, the VRM controller, and etc.

Step 2) If you have a shorted MOSFET as per step 1 above, use your multimeter to test all of the MOSFETs around the CPU. Set multimeter to resistance testing (lowest scale if manual multimeter - typically 200 Ohms), put black (-) probe on a ground point (e.g. shiny pad around a screw hole on the motherboard) and use the red (+) probe to measure resistance to all of the MOSFET pins on each MOSFET around the CPU. Report what results you get here.
That's all for now.