Will not boot, screen is black and Fans are loud and run all the time

[MDSteele]

HP Prodesk 600, [personal information removed]. will not boot and the screen stays black but the fans start running loud and fast in about 1 minute.  The cd drive lights up and goes off.  There is a green light on the motherboard

I have read mixed responses on other threads saying it's the motherboard or power supply or the hard drive.

How do I  determine which one it is that is if it is any? 

[itsmyname]

@MDSteele --  I have read mixed responses on other threads, saying it's the motherboard or power supply or the hard drive. How do I  determine which one it is that is if it is any? 

It is very-highly probable that it is one of those "any" choices, given that the symptoms arise as soon as you power-on the computer.

Given the speed and noise-level of the fan, I think that the POST (Power On Self Test) is failing to complete. To me, that rules-out the hard-drive as being the cause of the symptoms.

It is much easier to temporarily-connect a different power-supply than it is to replace the motherboard. So, I would try a different power-supply.

Replacing the motherboard is not easy, and definitely is expensive.  Also, your copy of Windows is licensed to the current motherboard. If you change the motherboard, you'll need to purchase a new license, adding to your costs.

[MDSteele]

It's the hard drive.

[itsmyname]

@MDSteele -- I doubt that it is the disk-drive.

When you power-on the computer, the POST (Power On Self Test) is started. The POST speeds-up the fans to 100%, and then should measure the temperature of the CPU and the motherboard. If the temperatures are "normal", then the motherboard tells the fans to slow down.  But, if there is a problem detected during the POST, the command to the fans to slow down is never sent to the fans. Thus, the fans continue to "roar" at full speed.

Also, if the screen stays black, then the POST has not completed.

Experiment: disconnect the data/power cables from the disk-drive. Then, power-on the computer. If the symptoms are still the same, it is NOT the disk-drive that is the cause of the symptoms.

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[MDSteele]

I was premature in stating it was the hard drive. I'm going to order a PSU and see what happens.

[MDSteele]

Installed power supply but it didn't change anything. Also recoated CPU with fresh Thermal Paste which was a good thing but made no difference!

 It must be the mother board.