pc turns off immediately after turning on

  • Thread starter Thread starter Nick
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Nick

I have a pc with a gigabyte motherboard which has been working ok for
the last 2 years.

Two days ago, it turned off on its own. Since then, when I press the on
switch, it briefly comes on for about 1 second and goes off (lights and
fans come on and go off after 1 second).

What could be wrong? Faulty switch? Power supply finished?
Motherboard/CPU faulty?
 
Nick said:
I have a pc with a gigabyte motherboard which has been working ok for
the last 2 years.

Two days ago, it turned off on its own. Since then, when I press the on
switch, it briefly comes on for about 1 second and goes off (lights and
fans come on and go off after 1 second).

What could be wrong? Faulty switch? Power supply finished?
Motherboard/CPU faulty?
Click to expand...
Could be any of those things. However it could just be that the CPU fan is
u/s. Although it may still be going round, if the speed sensor has stopped
working, your mobo will see the fan as not working and shut the system down
in the manner you describe. So, as they are cheap, try another fan on the
CPU heatsink and see what happens.

SteveH
 
"SteveH" said:
Could be any of those things. However it could just be that the CPU fan is
u/s. Although it may still be going round, if the speed sensor has stopped
working, your mobo will see the fan as not working and shut the system down
in the manner you describe. So, as they are cheap, try another fan on the
CPU heatsink and see what happens.

SteveH
Click to expand...

The BIOS has that fan checking feature in it, on some
motherboards. It might take several seconds for the BIOS
to figure that out, that the fan is too slow.

The Athlon64 and Pentium4 have THERMTRIP, which is an internal
detection of overheat. That signal is connected into the PS_ON#
signal logic that controls the ATX power supply. If the heatsink
falls off your processor, then shutdown in 1 second would be
about right. If the fan is stopped, it might take a bit longer
before the silicon die can heat up the heatsink enough to
trip the overheat detection.

The Athlon(32) didn't have thermal protection built into the
silicon die. But some versions did have a thermal diode, suitable
for monitoring with an external device. Some motherboards, like
the A7N8X series, had a small eight pin chip, that checked the
thermal diode output, and prevented burnout. Some other Athlon
motherboards used their hardware monitor chip to measure
the temperature, but there was no guarantee that the processor
would realize it was overheating in time. And lastly, there
were some other moherboards, that had no protection implemented
at all.

The power supply probably has its own protection logic, and
the complexity of that logic (what it is checking for) depends
on the price of the supply. Overvoltage, undervoltage, overcurrent
are some possibilities, and even internal PSU temps might be
another reason to shutdown.

If the motherboard was faulty, it might just stay off or
stay on. Turning on for a second and then turning off, is
a little too clever for a motherboard on its own :-) While
I can think of ways it might happen, I've never read of any
such cases.

I would check the condition of the heatsink/fan on the CPU
very carefully. Including, as SteveH has pointed out, that
the fan is still plugged in, and spins when the machine tries
to start. If the fan spins, but the machine still shuts down,
check the heatsink to see whether the clips are still
applying enough pressure to keep the heatsink in contact
with the processor. Sometimes a tab will snap and a clip
will come loose.

And if there is no effective thermal interface material between
the heatsink and the CPU, like you used a thermal paste that
dried out or pumped itself out from between the heatsink
and CPU, that would be another reason for the CPU to overheat.
The material that ships with an AMD or Intel processor, is
not likely to do that, while a hobbyist applied paste is
more likely to leave the scene on you.

Paul
 
Paul said:
The BIOS has that fan checking feature in it, on some
motherboards. It might take several seconds for the BIOS
to figure that out, that the fan is too slow.

The Athlon64 and Pentium4 have THERMTRIP, which is an internal
detection of overheat. That signal is connected into the PS_ON#
signal logic that controls the ATX power supply. If the heatsink
falls off your processor, then shutdown in 1 second would be
about right. If the fan is stopped, it might take a bit longer
before the silicon die can heat up the heatsink enough to
trip the overheat detection.

The Athlon(32) didn't have thermal protection built into the
silicon die. But some versions did have a thermal diode, suitable
for monitoring with an external device. Some motherboards, like
the A7N8X series, had a small eight pin chip, that checked the
thermal diode output, and prevented burnout. Some other Athlon
motherboards used their hardware monitor chip to measure
the temperature, but there was no guarantee that the processor
would realize it was overheating in time. And lastly, there
were some other moherboards, that had no protection implemented
at all.

The power supply probably has its own protection logic, and
the complexity of that logic (what it is checking for) depends
on the price of the supply. Overvoltage, undervoltage, overcurrent
are some possibilities, and even internal PSU temps might be
another reason to shutdown.

If the motherboard was faulty, it might just stay off or
stay on. Turning on for a second and then turning off, is
a little too clever for a motherboard on its own :-)
Click to expand...

Not at all, this is exactly what happens when the CPU fan speed sensor isn't
being read for some reason.

While
I can think of ways it might happen, I've never read of any
such cases.

I would check the condition of the heatsink/fan on the CPU
very carefully. Including, as SteveH has pointed out, that
the fan is still plugged in, and spins when the machine tries
to start. If the fan spins, but the machine still shuts down,
check the heatsink to see whether the clips are still
applying enough pressure to keep the heatsink in contact
with the processor. Sometimes a tab will snap and a clip
will come loose.
Click to expand...
Except I didn't say that. I pointed out that if the speed sensor on the CPU
fan fails, then most BIOS will stop the system to prevent CPU overheat. This
is something I've seen and fixed by the simple expedient of replacing the
fan several times. I've also seen this happen where a person building a PC
has accidentally plugged the CPU fan into the wrong header.

SteveH
 
Just tried a few things one of which gives a clue.

If I unplug the ATX12v power voltage (I think this goes to the cpu) and
then switch on, the power and fans stay on although the pc does not
boot up for obvious reasons.

So it could be something to do with the cpu (heatsink, fan, power
supply, processor)?
 
Nick said:
Just tried a few things one of which gives a clue.

If I unplug the ATX12v power voltage (I think this goes to the cpu) and
then switch on, the power and fans stay on although the pc does not
boot up for obvious reasons.

So it could be something to do with the cpu (heatsink, fan, power
supply, processor)?
Click to expand...
It could still be the CPU fan. If you have another fan avialable (it can be
a case fan, if it has the right connector), plug it into where the CPU fan
plugs into the mobo, and then try switching on. If it stays on, it means I'm
right and your CPU fan needs replacing.
If it does indeed stay on, tap the delete key (or whatever you need to do to
get into your BIOS and then switch off again (you may have to hold in the
power switch for 4 secs). Go and get yourself a new CPU fan and fit it, job
done!
If the above doesn't work, then you will need to start investigating other
possibilities. The above could be the cheapest fix however.

SteveH
 
Most likely the PSU has failed. If not, then the motherboard would be the
second most likely culprit.
 
DaveW said:
Most likely the PSU has failed. If not, then the motherboard would be the
second most likely culprit.
Click to expand...
In my experience, the most likely culprit is the speed sensor in the CPU
fan, the failure of which often causes the symptoms the o/p describes. But
as you say, could be the mobo, PSU etc.

SteveH
 
I just borrowed a cpu fan off another pc and tried it and there is no
difference. So its not cpu fan.

I am going to replace the power supply next. Watch this space.
 
Nick said:
I just borrowed a cpu fan off another pc and tried it and there is no
difference. So its not cpu fan.

I am going to replace the power supply next. Watch this space.
Click to expand...

Is the 4-pin power connector from the power supply attach properly
to the motherboard?
 
Yes, the plug slots in correctly on the motherboard.

btw its a Pentium4 2.4, Gigabyte motherboard. It has been working fine
for 2 years. Then it crashed twice (rebooted itself). Then 3rd time it
rebooted but did not get past all bios checks. Then after that there
was no display and it switches off one second after being switched on.
 
Just replaced power supplu. No difference.

Swapped out CPU - no difference.

Clutching straws now.
 
Nick said:
Just replaced power supplu. No difference.

Swapped out CPU - no difference.

Clutching straws now.
Click to expand...
Any more bits you can swap out? Video card, memory etc. Have you tried it
with no memory?
Must admit, it is starting to look like the mobo. If you could get into the
bios, which you presumably can't, I would say turn off the fan speed
monitoring and see what happens.

SteveH
 
look for bulging leaking or discolored capasitors, i think you said it was 2
or 3 years old right?
 
Ok, I removed everything and and then started putting back together.
HD ok, CD rom ok, memory ok - powering staying up. As soon as I put the
AGP graphics card in, back to old habits...power goes off after 1 sec.

Swapped agp card for another and still same. Tried putting a pci
graphics card in - same.

In fact if I put anything in the expansion slots, the symptoms come
back. If I have nothing in expansions slots, power comes on ok and
stays on, HD light comes for a while but cannot tell how far boot is
getting as no display.

So obviously there is something to do with expansion slots.
 
No option but to swap motherboard now. Will Win XP work as before from
existing HD or do I have to install again.
 
Nick said:
No option but to swap motherboard now. Will Win XP work as before from
existing HD or do I have to install again.
Click to expand...
It may, if it's close enough, but you won't have removed all the old drivers
under Windows. Personally I always favour a reinstall.

SteveH
 
But I dread losing all the software that is installed on it.

Can I re-install/repair without losing my software and data?
 
Nick said:
But I dread losing all the software that is installed on it.

Can I re-install/repair without losing my software and data?
Click to expand...
You may be able to. You'll just have to try it. If you can get into XP,
you'll just need to change the drivers and it should all work ok.

Alternatively, when the XP install gets to the point where it wants you to
select a drive/partition, you may get an option to repair an existing XP
install, if the installer perceives your existing installation to be broken.

Otherwise find a friend who is wiling to let you attach your existing drive
to their PC and copy your data to cd/dvd

SteveH
 
SteveH said:
You may be able to. You'll just have to try it. If you can get into XP,
you'll just need to change the drivers and it should all work ok.

Alternatively, when the XP install gets to the point where it wants you to
select a drive/partition, you may get an option to repair an existing XP
install, if the installer perceives your existing installation to be broken.
Click to expand...

The option to repair shows if there's an existing XP system on the hard
drive, not that it's 'broken'.
 
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