Dell desktop shuts down and restarts on its own

So I have a client that have a continious problem with his fairly new desktop (about 1 year old).  The computer shuts down on its own and restarts.  No BSOD so no dmp files to analize.  
The Dell quick diagnostics do not give any failures and all components test pass.  I did re-seat all the RAM modules.
I had Dell to come out and swap the motherboard and a power supply.  The system ran OK for a day and shut down again today.
The only thing I noticed the aux led on the motherboard is yellow.  Not sure if that should be green or maybe its ok.

The system is still on warranty but its a horrendous experience to deal with Dell.  

Any advice appreciated.
Thank you.
LVL 17
Tiras25Asked:
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jamietonerConnect With a Mentor Commented:
Have you tried plugging the system into a different surge protector and a different outlet? Power fluctuations in the ac line can cause the system to shutdown and if the bios is set to last known power state the system would appear to just restart itself. Another way to test this would be to change the power option in bios to stay off instead of last known state. I have seen a similar issue a few times with clients, the clients system would constantly restart at the location, but when on my workbench it would run flawlessly. Most of the time it was just a faulty surge protector but a couple times it was a more serious electrical issue.
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David-HowardCommented:
Most motherboard LED's should be green. Since the power supply and motherboard were replaced I would suggest the following. Make sure that the CPU is properly seated and that it is not overheating. Normally when systems overheat they will shut down after up and running for about the same amount of time.
Dell has a good web site for troubleshooting. You might head over there and look up the customer's make and model. Then you can take a look at that yellow motherboard LED.
I'm a little surprised that a new power supply and/or motherboard didn't correct this issue though. But as I'm sure you know, new doesn't mean that it can't be bad.
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edbedbCommented:
There are normally only 3 things that will make a system suddenly shut down. The PSU, motherboard and the display adapter.
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Tiras25Author Commented:
This is an Optiplex 755.  I looked around the Dell's support and did not see anything regarding that yellow AUX LED. I am suprized too the motherboard replacement did not correct the issue.
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edbedbCommented:
I would try re-installing the drivers for the display adapter.
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Tiras25Author Commented:
The thing is the system event log doesn't show any errors either.
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agengler11Commented:
These mobos have a yellow heartbeat LED so no worries there. Do you have a power surge protector on the unit? Is Windows patched with all updates? Reseating the ram was a good start but this time try putting one in at a time and running the PC. If it runs good for a bit then add a second etc.. I suspect bad ram.
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David-HowardCommented:
Were you able to look at the cpu seating yet? As I said, a system that shuts down on it's own after about the same amount of up time usually means an overheating somewhere.
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Tiras25Author Commented:
Didn't get to CPU yet.  I don't think it shuts down after same amount of uptime.   Its completely sporadic.
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mccrackyCommented:
I know it may not be pretty, but put the onus where it lies.  Make Dell honor the warranty.
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David-HowardCommented:
If it's random and the motherboard and power supply have been replaced the problem is narrowed down quite a bit. RAM, CPU, Video or even possible short between the motherboard and casing (which I doubt).
At this point I would start replacing one RAM stick at a time (as mentioned above). And test. One stick only and run for effectiveness. If it fails, remove that stick and replace, so and so forth. But also as mentioned above, if this were my system (and I'm a tech), Dell is honoring this warranty. It's their obligation to troubleshoot and repair this system. Not yours. And besides, you could run the risk of ruining the warranty if they find out you've had your hands in the system.
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PURESKILLZZZConnect With a Mentor Commented:
Its more than likely a heating or power issue . A UPS is a good start . Also try blowing machine out with air for dust ... More cooling fans also help as dell is cheap with those.
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Tiras25Author Commented:
The machine is very clean inside/out.  I will check the power tonight though.
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amodeoCommented:
Light is always amber on the motherboard of a Dell optiplex 755, check and see in the bios for "restart computer after power failure" and if that is on turn it off... That will tell us what is going on next time you experience this issue... The computer won't restart if it thinks it is experiencing a "power failure" and then we can look forward to see if you need some kind of power conditioning device or whatnot.

-Joe
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kaskhedikar_tusharConnect With a Mentor Commented:
Hello,

Firstly,please check the power supply & earthing & UPS (if you installed) related with electrician.
Then go for computer hardware.

Regards,
Tushar Kaskhedikar
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nobusCommented:
for troubleshooting, disconnect devices and cards, or disable tehm from the bios.
i mean : all extra disk, and cd drives, remove extra ram sticks - add on cards
 disable sound and network from bios to test !
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David-HowardConnect With a Mentor Commented:
I had another thought on this. Has this system been moved to another area/power source for troubleshooting? I'm wondering if there isn't something on the circuit that it is currently plugged in to that may be causing the intermittent shutdowns.
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Tiras25Author Commented:
Looks like I found a cultpit last ngiht.  The computer plugged into the APC UPS and the USP into the power strip that is plugged into the wall jack.  
I noticed the power green light on APC is off even though it still holding a power.  So I removed the UPS and plugged directly into the power strip.  So shut downs so far.   Most likely that UPS was causing  power spikes that shuttnig down a computer.
I will report in a bit.
Thanks!!
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nobusCommented:
an UPS causing power spikes...lol
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Tiras25Author Commented:
Exactly :-)
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David-HowardConnect With a Mentor Commented:
Glad to know that you figured it out. I assumed a possible faulty circuit. But should have asked if you had anything between the pc and the outlet.
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Tiras25Author Commented:
Hopefully that's the problem.  I will wait through the day today to confirm.  No shut downs so far.
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rajesh_rhceCommented:
hi
 just chk power supply wattage,

if power supply wattge is good(450w) then u chk with out hard disk system will run properly,then u have to  install new os.
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