I have an HP pavilion M9040N tower that has power (green Light on) but shuts of in 1-2 seconds after turning on the power button( blue light) on top. Tried removing various components with no change until I took out the processor (Intel Q6600). Now the blue light stays on. No history of this computer tower. Appears stock condition, clean and lightly used. Gut feeling that the processor or motherboard is defective.How do I differentiate where the problem is

Try removing the 512 MB memory to see if it does work with the 1GB stick only.

It's not the power supply that does have a problem. It has something with your memory stick.

Sounds like your power supply has gone bad. You will need to get a new power supply. They can be found online, or in most electronic retailers.

Hi.
Before you go and get a new motherboard, first let's try to do a hard reset.

Sometimes, hp and Compaq computers bios will lock up, causing this condition.

To do the hard reset, first make sure all the cords are connected to the points where they belong.

Now, unplug your ethernet cable, and your power cord.

This stops all power from coming into the machine.

Next, press and hold the power button in for 30 to 45 seconds.

This causes the capacitors on the motherboard, and the ones in the power supply to dissipate, and returns them to a ready state.

After doing that, reconnect just the power cord and attempt to power on the computer.
If it boots, then it was the bios glitch, if not, then you have a faulty motherboard, that or you have a device installed the wrong way, so that when the system boots up, it calls for the power to that device in an unusual way, then the system will shut down to prevent damage.

If it does boot up, then you can reconnect the ethernet cable back, otherwise there is no sense of hooking it back up as you would be replacing the motherboard anyways.

I hope you have good luck with this, and have a good day.

In my case it turned out to be a weak power supply. It would work sometimes and not others. If stone cold, it would not boot. If I unplugged 1 thing, like the hard drive or the CD Roms, it would boot. Tested the old power suppy and it seemed to pass. But once I connected a new power supply, the computer has worked fine ever since.

Pull the power cord out and press the power button on the PC to drain the power for 10 seconds. Then plug the power cord back in and turn the computer on. If still does not work, then the power supply may be blown out and needs replacement.

Sorry but the chipset on the motherboard failed, you will have to replace the board.

Psu or motherboard fault and use reginout

verify if at rear is a button with 1 and 0 position
that must be on 1

try this link.....it has the steps you need to follow
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01732674&cc=us&lc=en&dlc=en

I would fathom a guess that the motherboard has been damaged, if it is the PS then you can see by taking it into a service center to have the output checked 3 v 5 v 12 volt good luck.