Computers & Electronics

How to tell if the MB or the CPU fails when the PC can't power on?

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  • Feb 24th, 2010 10:25 pm
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Deal Addict
Feb 12, 2004
1672 posts
231 upvotes

How to tell if the MB or the CPU fails when the PC can't power on?

I've had this HP media center PC for 2 years. It could not power on on the weekend. I opened the case and found that neither the CPU fan nor the PSU fan runs. I'll just buy a MB or MB + CPU, and keep the case and other parts. But the problem is I don't know if the MB, CPU or both of them failed. Is there a way I can figure that out?
10 replies
Deal Addict
Sep 15, 2004
3309 posts
147 upvotes
Dollars to doughnuts that it's a power supply failure... (usually the 12V line) unplug the heaviest power drain (the hard drive) and hit the power on. LED light's up, video anything? (nothing) points more at the pwrspply. CPU failure would give some video err... MB failure might give a diag LED.
Deal Addict
Feb 12, 2004
1672 posts
231 upvotes
It's not a PSU problem. I can switch that PSU to my other PC, it works fine. Also use another working PSU on this dead PC, doesn't power on either. One of the PSU has a LED on it. If I attach the power cord, the LED is on. But when I press the switch on the PC, that LED on the PSU becomes off, and nothing happens: no beep, no fan, no screen.

It has to be the MB or CPU problem. But I don't have another MB with the same type of socket to test them.
Deal Expert
Aug 2, 2004
38392 posts
12019 upvotes
East Gwillimbury
Did you try another power supply?

If you are positive it is not the power supply, then I would say it is more likely the motherboard. Since you are not getting any POST (beeps) messages
Deal Fanatic
Mar 21, 2002
6827 posts
1482 upvotes
Manitoba
If it's not the PSU then it's the MB. CPU failure is *extremely* rare.
Deal Addict
Oct 4, 2009
1541 posts
7 upvotes
It's odd that neither fan is running. Newer HP motherboards don't usually fail that way. Usually the BIOS goes in them. Disconnect everything but the powersupply, processor and heatsink, and RAM. Disconnect ALL the connectors. Try jumping the power switch pins on the header with a small screwdriver or butter knife. If that fails, pull the board out and set it on something non-conductive. Repeat the experiment. If it fires up outside the case it's the metal bracket under the processor shorting out the board. We just saw that happen on the same vintage HP tower this past weekend.
Deal Addict
Sep 29, 2009
2028 posts
545 upvotes
MTL
You should try to turn it on with the case open to see if the fans flinch. Flinching means there was some 12V power going through but cut out (possibly due to load or short). No reaction at all would mean the PSU is toast, or no power on signal from the motherboard (if it's disconnected).

Power LED on the motherboard usually indicates presence of standby power. No LED = bad power supply. LED = could be bad PSU or motherboard.
Deal Addict
Sep 15, 2004
3309 posts
147 upvotes
some heavy load might be holding the system down... (possibly an internal short.. even inside a chip) make sure you have NO hard drive in the system when you test. Shorted MB is the next culprit, but you're going down a road guessing. Get some replacement parts and maybe you'll be able to make some sense out of this issue.
Deal Addict
User avatar
Aug 19, 2005
2369 posts
569 upvotes
York
I've yet to see a CPU fail in all my years dealing with PCs. The motherboard is possible, but sounds like the power supply to me. Go and order a good wattage brand name power supply to avoid any issues in the future.
"It's how you deal with failure that determines how you achieve success."
Deal Fanatic
Sep 4, 2009
6575 posts
726 upvotes
Try running the system out side of the case in case of a short. I thought initially it was PSU problem to be honest.

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