PC problem

I had the same PC for over 5 years. It stopped working while I was using it.

I started taking everything out and started with just PSU MB and CPU only. Here is where im confused... Motherboard will boot and stay on when I unplug the 4pin 12v connector.

So i took out the CPU and here is what happens...
* PC boots and stays on when CPU is out 4pin connector is in
** PC starts for a quick sec. but then goes off when CPU is in & or 4pin connector is in.

Whats causing my pc to do this?

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Clarification Request
I don't much to work with here.

It sounds as if you have figured out it's either the PSU, motherboard or CPU but want some test to figure out which.

Sadly the CPU test is to put the CPU into another working system.

The PSU test, most of the time is to use a Volt Meter and power up the PSU using the GREEN WIRE METHOD then check the voltages. HOWEVER this is a partial test since it can't tell you if the PSU works under load. For that we swap in another PSU.

Now that we have tested 2 of the 3 parts we can, at this point guess if it is or is not the motherboard.

--> Worth noting at 5 years is the BAD CAPS issue. Google BAD CAPS so you know what to look for.

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Well from what i understand the MB can not work without the CPU right?
But the PC fan & the CPU fan are both working when I hit the power button, so im guessing it can work on new PCs?

Anyways so it looks like (PSU+MB) together work... then I add the (CPU or the 4pin connector) and it cause the PC to boot for a sec. then stops
My CPU looks ok. I dont think anything happen to it yet trying to start my PC with (PSU MB and CPU) shows that having the (CPU in or the 4pin connector in) makes it work for a sec. then stop

From what I read it does that to save my hardware from any damage.
I dont have any tools to test each one of these 3 parts alone and I dont have another PC to switch parts with to test them that way.

*Maybe its clear that its my CPU but I'm thinking if a hardware break in a PC it should be easy to see the damage on it or some sort of a burn of some sort?
**But then again can it be the PSU and it only stops when the CPU is added or the 4pin connector is plugged in?

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Well from what i understand the MB can not work without the

"Well from what i understand the MB can not work without the CPU right?"

Define "work"! For years we could and still can do some minor diagnostic work with just the motherboard and PSU laid on on cardboard and a jumper to power up these 2 parts. It doesn't tell us much but a power up can tell us that it's not a full on dead short and the power up circuit (that one that runs from the power on switch to the GREEN WIRE on ATX PSUs) "works."

The 4 pin connector is interesting but is not part of our diagnostics.

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I mean by work... the power light is on the PC and CPU fan are working and the PSU keeps on going.

If I add the CPU and unplug the 4pin it works
If I take the CPU and plug the 4pin back in it works.

If both 4pin and CPU are in it stops a sec. after it starts

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Sounds like you are down to 3 parts.

Let me share the other tech shop rule. When you are down to some number of suspect parts and you can't tell which is bad, change the cheapest part first. It may not be the right one but it was the cheapest part. Proceed to the next cheapest part until the system works.

Hope this helps you along.

We've already covered the Volt Meter GREEN WIRE test, BAD CAPS yet you didn't tell how your parts fared.

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When i looked into this more. I found out that its a very clear sign that its my CPU. My question now is...

I was looking into buying an (i5 processor) and my CPU right now is (Intel Core 2 Duo processor)... Are the sockets and pins the same or do i need a different motherboard for (i5)?

My PC is...
HP Pavilion m7680y customizable Media Center TV PC
? Motherboard: is Asus (I can get the info for it if its important)
? Processor: Intel Core 2 Duo E6600 (2.4GHz)
? SDRAM: 2GB DDR2-667MHz dual channel (2x1024)

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That motherboard

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