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Intermiitent CPU or Motherboard?

  • CPUs
  • BIOS
  • Hewlett Packard
Last response: in CPUs
September 21, 2010 11:31:46 AM

I have an HP m9402f that frequently fails to boot. I have the diagnostics display enabled. When it fails it displays the BIOS info, but stops just short of displaying the CPU info. No beeps. I have tried a different PSU and reseated the memory many times. When it does boot, the HP diagnostics (from the BIOS) indicates everything is fine. I think it is the CPU intermittently failing, but I know this is supposed to be rare. Does the MB & BIOS have the ability to display the BIOS screen if the CPU is bad? What should I do next? Thanks.

More about : intermiitent cpu motherboard

a c 172 à CPUs
September 21, 2010 12:48:48 PM

If the CPU is bad, you will get a dead screen.

Work through our standard checklist and troubleshooting thread:
I mean work through, not just read over it. We spent a lot of time on this. It should find most of the problems.

If not, continue.

I have tested the following beep patterns on Gigabyte, eVGA, and ECS motherboards. Other BIOS' may be different.

Breadboard - that will help isolate any kind of case problem you might have.

Breadboard with just motherboard, CPU & HSF, case speaker, and PSU.

Make sure you plug the CPU power cable in. The system will not boot without it.

I always breadboard a new build. It takes only a few minutes, and you know you are putting good parts in the case.

You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.

If no beeps:
Running fans and drives and motherboard LED's do not necessarily indicate a good PSU. In the absence of a single short beep, they also do not indicate that the system is booting.

At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.

This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.

If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card.

Silence or long single beeps indicate a problem with the memory.

Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.

Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.
September 21, 2010 12:54:47 PM

By the way, you wont hear any beeps if no PC Speaker is connected (on most motherboards)
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September 21, 2010 11:57:07 PM

Thanks to all for the replies.
I verified the PC speaker worked (I could see it - it is soldered to the MB) by removing ALL RAM and booting. It is extremely high freq but it does beep. I definately prefer the real speakers of old !!!!

After re-installing the RAM, the PC boots maybe 1/2 the time.

Please note the sequences below:

When it fails:
Display has the BIOS info (version, etc).
No beeps.
System does nothing else.

When it works:
Display has BIOS info (version, etc).
Display has processor info (version, etc).
Pause, then memory & HD/CD info.
Then Windows boot.

Please note that when it fails it does not get past displaying the CPU info.
I know the processor is running since it would not display anything otherwise,
but if it is working properly why does the BIOS not identify it?

I have tried 2 other power supplies to no avail.

I do not hear a POST beep like my old Dell computers if it works or not (this is an HP m9402f).

I would reflash the BIOS, but cannot find even the current version anywhere (I did look at the HP website).

a c 158 à CPUs
a b α HP
September 22, 2010 12:57:36 AM

The guy I work with had that same computer and problem last year. I looked at it with him and saw it was out of warranty by a few months but I called HP anyways. We went thru all the diagnostics I had already gone thru again and it still wouldn't boot up so HP replaced it under warranty. I was amazed I tell ya.