# MB starts boot only with 4 -pin CPU plug removed



## CREnvy (Sep 10, 2010)

A system I built and which has been running for a few months has suddenly decided it wants to run no more. The symptoms are that the board LED comes on, the CPU fan runs, slowly, but case fans don't and the HD never spins up. I have tried a number of things recommended here and will do the bench test soon, but I did find one curious symptom that I wanted to share in case it jogs someone's memory and they have a thought that might save me some time in troubleshooting.

While trying to solve the problem by unplugging components, memory, etc. I notice that if I just remove the 4-pin CPU power plug (regardless of what else is or isn't plugged in) that the machine comes to life. Of course, without the CPU it never gets to BIOS, but it appears the problem is associated with that power port.

Ring any bells?

Thanks.


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## Wrench97 (May 10, 2008)

It's just the fans running, without the 4 pin plugged in it's not trying to boot.

What brands and models are you running for the PSU, Motherboard, CPU, Ram and Video card?

Have you tried removing all but 1 ram stick and trying to boot?


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## CCT (Mar 21, 2009)

IF supplying the extra power needed by the cpu stops the unit, you have 3 possible sources for the problem that I can see;

- bad psu

- bad mobo or mobo mount problem

- bad cpu


PSU can be checked with a tester

Mobo can be removed (an out-of-case benchtest) and check for blown caps or burn marks

CPU checking is problematic, since who wants to try a bad cpu in their comp and you'd risk destroying a new cpu in your mobo


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## CREnvy (Sep 10, 2010)

Did the bench test procedures. PSU seems fine, visual inspection of the MB turned up nothing. Re-seated the CPU and symptoms remain. I _know _that I have another CPU I can try, but dang if I can find it! No joy today.


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## Wrench97 (May 10, 2008)

Did you clear the CMOS using the jumper on the motherboard?

To tell if the PSU is fine you need a PSU tester or a digital voltmeter, or a spare PSU to swap in. Sight, sounds and feel won't tell you>  Test Your Power Supply 

Would still help to know exactly what it is your working on?


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## CREnvy (Sep 10, 2010)

Yes, did all that, unplugged everything, 1 stick of RAM, etc. Used a PSU tester (of course, it doesn't load it, just checks the voltages, but the PSU is only a couple months old).

It's an Intel dual core (forget the model no.), 2 IDE HDDs, 1 DVD-RW, 4 sticks PC4200 (2 GB total), MB is an Intel DG965OT, PSU is Antec EA 380D (380W, which is adequate+30% according to the power calculator elsewhere on this site).

Since I last posted I found an LGA775 Celeron dual core for 7 bucks on eBay that will do fine for testing.


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## Wrench97 (May 10, 2008)

It's probably not a CPU(always the last suspect on the list) since you have a different reaction when it's powered up vs not powering the CPU.

Do you have a motherboard speaker?
If yes and you remove all the ram and attempt to boot does it beep?


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## CREnvy (Sep 10, 2010)

Hmm, that logic seems backwards to me. If the symptom has to do with powering or not powering the CPU then that seems to point to the CPU, to me. If it were the motherboard then it would seem the symptom would remain in either case. Anyway, when I plug in the CPU power only the CPU fan turns, slowly. When I unplug it then the case fans start up as it it's preparing to boot. 

Yes, there is a motherboard speaker, but it doesn't make any beeps. I think the process isn't making it that far. Not even when all RAM is removed.

Fortunately, this is not my main PC, so I can wait for the "new" CPU to arrive to test with it. I figured if I bought the new one I'd for sure find the one I have stashed somewhere! :4-dontkno


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## Wrench97 (May 10, 2008)

You can take the CPU out and have only the fans run.


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## CREnvy (Sep 10, 2010)

When I do that nothing happens. No fans, no beeps, nada.:sigh:



wrench97 said:


> You can take the CPU out and have only the fans run.


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## CREnvy (Sep 10, 2010)

Finally got another CPU to try. No change, so I guess we can blame the motherboard.


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## Wrench97 (May 10, 2008)

What motherboard is it?


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## CREnvy (Sep 10, 2010)

wrench97 said:


> What motherboard is it?


Intel DG965OT. See up-thread.


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## Wrench97 (May 10, 2008)

Check the caps on the board> Check the Capacitors on the motherboard for any signs of leakage, domed tops, missing cans.> Badcaps.net - How To Identify


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## CREnvy (Sep 10, 2010)

Already did all that. Can you not see the previous posts in this thread. :tongue:



wrench97 said:


> Check the caps on the board> Check the Capacitors on the motherboard for any signs of leakage, domed tops, missing cans.> Badcaps.net - How To Identify


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## CREnvy (Sep 10, 2010)

I will try to post some close-ups of a couple of questionable caps today or tomorrow. They really don't look anything like the examples I've seen, but hey it's worth a try. One other symptom: if I leave the MB to sit for 2 to 3 days (regardless of whether the PSU is switched on or not) it will boot! It seems to operate normally, I can restart multiple times, BUT if I do a cold restart (shutdown), it will not boot again. :4-dontkno



wrench97 said:


> Check the caps on the board> Check the Capacitors on the motherboard for any signs of leakage, domed tops, missing cans.> Badcaps.net - How To Identify


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## CREnvy (Sep 10, 2010)

All of the example bad caps photos I've seen don't seem like what I'm seeing on my MB. So, here are a couple of photos that the experts can take a look at. 

There are caps with a cross-indentation on the top and crosscaps.jpg is an example of those. They all seem flat on top. The other kind have a K indented on the top and are all branded Sanyo. See kcaps.jpg. Since these are not doming up across the whole top I'm wondering if they are really bulging or that is just how they are made?

http://www.tomochka.com/images/crosscaps.jpg
http://www.tomochka.com/images/kcaps.jpg

TIA.


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## Wrench97 (May 10, 2008)

They look ok, the creases are actually breakaway points if the cap were to fail the lines that form the x are supposed to be a weak point for it to break open and relieve any pressure inside.


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## Stu_computer (Jul 7, 2005)

> While trying to solve the problem by unplugging components, memory, etc. I notice that if I just remove the 4-pin CPU power plug (regardless of what else is or isn't plugged in) that the machine comes to life. Of course, without the CPU it never gets to BIOS, but it appears the problem is associated with that power port.


It does _appear_ to be the problem, and occasionally it is due to a bad connector, but normally the problem is elsewhere.

The 4-pin CPU power plug, aka ATX12V (and the 8-pin version), is strictly used by the CPU and doesn't power any other components. This is due to the large instantaneous current demand during power up. Though a CPU typically uses about 4~8Amps during normal operation it can draw up to 20Amps during power up. Power wise thats 20x12=240, and thats like having two 100 Watt and a 40 Watt light bulb all drawing from the same source point. There will be a lot of heat produced.
Rather than have to try and provide a large enough circuit from the main 24-pin power connector to the CPU it is better to provide a closer dedicated connection so that heat dissipation and voltage drop is not a problem for the circuit board design.

If a power supply (PSU) cannot deliver sufficient start up power to the CPU during boot up then the CPU will fail to start and the system will hang.
The likely problem in this type of senario would be an underpowered or age degraded ATX12V circuit in the PSU is not able to meet instantaneous current demand during boot up and that PSU needs to be replaced with a better PSU.
In a worst case senario, (ie: a low quality PSU), it dumps dirty power into the circuit which in turn may destroy the CPU. Both the PSU and CPU would need to be replaced.


The mainboard is controlled by the chipset, not the CPU, for the Intel DG965OT it is the ICH8 chip that does the power management control.
In the case where no CPU is in the socket the ICH8 detects this as a possible CPU signal error and immediately issues a platform reset (cold boot start). The result is the board appears to power up, and if you now press the power button for more than 4 seconds the board will power off.

When the CPU is in socket but the system doesn't power up properly and the PSU is not faulty or underpowered, and there are no error beeps (ie: such as should happen when no ram installed), then it is either due to:
1. system has entered ACPI S4 sleep state on power up.
2. CMOS (vram) memory may have been corrupted, or wrong settings used during BIOS setup.
3. if the CMOS battey is too low/dead the RTC will fail and system may hang.
4. a fault bit set/stuck in the ICH8, (or the ICH8 itself is faulty).
5. a possible thermal problem, or defective/dead CPU.
6. a defective mainboard.

Possible solutions:
1. press start button for more than 4 seconds to power off, wait 30 seconds then press again briefly to power up as normally would be done to power on.

2. if CMOS data about the CPU or RAM is incorrect or becomes corrupt then the system will hang. The CMOS must be reset as per the method described in the mainboard's manual.

3. replace the CMOS battery, (the flat round button battery located on mainboard).

4a. toggle PSU power switch (or unplug AC cord) three times in a row to force the chipset to reset itself.

4b. If system still fails then it may be that the BIOS ROM cannot be loaded (has been corrupted) and the system will hang. This may default to seek an alternate source for the ROM (re: bootblock) such as a floppy drive or CD drive connected to the system. Consult manual for methods used to reload a new ROM.

5a. if there is a thermal problem the system will hang. Remove CPU and check socket for excessive heat/burn indicators (ie: socket warped or scorched). Reseat the CPU, apply new paste and carefully mount heatsink.

5b. temporarily replace CPU with a known good CPU, if system starts then original CPU was defective. Alternatively try the suspect CPU in a known good mainboard, if this system does not start then suspect CPU is defective.

6. if the CPU tested okay as per #5 above then some component(s) on the circuit board has caused a hardware failure. An up close visual inspection may reveal a faulty component such as a bad capacitor, a burnt resistor or IC casing, also check that BIOS chip is fully seated in it's socket and RAM modules are fully seated. If the component cannot be replaced then the mainboard must be replaced.


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## CREnvy (Sep 10, 2010)

How does this jive with the fact that usually after the system has sat for a couple of days it will boot to the OS and seems to run normally? It will even restart (warm reboot) and keep going. However, if I do a cold boot it again will not boot.

I was hoping that this additional symptom would narrow down the cause a bit.


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## Stu_computer (Jul 7, 2005)

The chipset monitors whether it is a cold boot or warm boot. The BIOS POST code checks this status to determine if it is to do a full initialization (cold boot) or a modified initialization (warm boot).

On a cold boot the BIOS has no idea what OS is to be used so it initializes all the components it detects. When a modern OS is loading it takes control of most components (loads their device drivers) and the BIOS is no longer involved in system operation.

On a warm boot the BIOS bypasses the components that the OS now contriols and only does a limited POST check (ie: power ok, thermal ok, set components it still controls, etc).

Since it takes a few days before the system will cold boot again it is unlikely its a thermal issue as that would dissipate in a few hours at most.

To check if it is a power issue unplug the AC cord and press the start button for 30 seconds to drain residual power then try to cold boot again.

To check if it is a component hang boot to a DOS floppy (or DOS CD) or simply disconnect the drives, then boot system and leave it sit for awhile; do a few reboots then power off and see if it will cold boot again.


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