Powers up but no activity; no POST, no beeps, nothing.

[sgodun]

Pressing the power button yields the power LED and all the fans inside turning on, but that's it. No beeping, no POST, screen remains asleep. Have tested and reseated every cable, the hard drive, and RAM to no avail. Cleared the CMOS (both via the jumper and by pulling the battery). I even replaced the CPU with a known-good one and got the same nothing. All of the components (RAM, hard drive, etc) work fine on other computers. I went through HP's mostly-useless "virtual agent" to no avail. I attempted BIOS Recovery but nothing happened (no beeps or anything, doesn't go into BIOS Recovery Mode).

Attn HP: If you're going to reply to this, please don't tell me to go to private messaging. Post your reply here so that everyone can benefit.

[Grzwacz]

Greetings,

Welcome back to the forum.

I am not a HP employee.

You have done some real good troubleshooting! 

Did you reseat the 24 pin MB power cable and the four pin CPU power cable?

Test the following hardware categories separately.

Only connect a monitor to the PC.

Try different a graphics card if possible or try onboard graphics if you use a discrete graphics card now.

Have you tried powering on with no SATA drives (including the optical drive) and then try booting with no RAM connected to the MB? See if you can get any POST error messages or beep codes.

I would also disconnect front USB (including the card reader) and try to boot the system.

It appears the MB may have failed or the PSU is failing.

Regards

[sgodun]

Yes, I reseated all of the cables. I also tested the power supply with a handheld tester and it is putting out the appropriate power and "power good" signal.

I have tried only connecting a monitor.

The computer has never used a discrete graphics card; on-board video only. But I have tried with a known-good graphics card and got the same effect.

I have tried with no SATA drives (hard drive or DVD drive). I have also tried booting with no RAM installed. The computer doesn't beep (or do anything else). I've even tried booting without the CPU (just to see if anything different would happen) and still get nothing.

I have disconnected EVERYTHING from the motherboard, except for power and memory and video. Nothing.

I am inclined to think that it's a hopelessly damaged BIOS but I have no way of verifying this.