# When CPUs Die Do the Fans Still Come on? **URL:** \ **Category:** Factual Questions\ **Created:** [August 10, 2004, 6:11pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231 "2004-08-10T18:11:26Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 10, 2004, 6:11pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/1 "2004-08-10T18:11:26Z")
I ask because something has gone dreadfully wrong with _my_ computer. It made the high pitched beeeeep noise that it makes when the CPU gets too hot. I have the alarm set to go off @ 140F. The default alarm is set to 158F. (Previously, while experimenting with fan speeds, I heard this noise before. During this experimenataion process, I ended up adjusting the fan speed while it was running. It was sufficient to keep the machine running, but it was insufficient to start the machine.) It’s now behaving similarly to the way that it did when the fan speed was set too low to startup- except that I’m not getting the beeep that says it’s too hot. This time, right now, the fans initially began to turn a bit then quit. Now they don’t even begin to turn. I have a green light on the mobo showing that I have power at least to that LED. When the machine crashed, the box was open and it was in an air conditioned room. The machine had run well previously each of the previous days while it was in this condition and in this room. **If, by some odd chance, the CPU burned up, how would the machine react? Would it behave the way that it is behaving?** (Not wanting to do anything but light up hte LED power indicator.) I feel sure that there’s some sort of a hardware malfunction. "Im just trying to decide if it’s the CPU, mobo, power supply or ‘other’. I have a lot of work to be doing on this machine (posthaste of course) and no other computer in house with SATA connections to retrieve the data in the mean time. ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 10, 2004, 6:38pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/2 "2004-08-10T18:38:46Z")
Vanity search bait: **pestie** **engineer\_comp\_geek** ---
**Author:** ![counsel\_wolf](https://avatars.discourse-cdn.com/v4/letter/c/65b543/32.png) [@counsel\_wolf](https://boards.straightdope.com/u/counsel_wolf)\ **Post date:** [August 10, 2004, 6:54pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/3 "2004-08-10T18:54:39Z")
> [@SimonX](#): > > **If, by some odd chance, the CPU burned up, how would the machine react? Would it behave the way that it is behaving?** (Not wanting to do anything but light up hte LED power indicator.) > > I feel sure that there’s some sort of a hardware malfunction. "Im just trying to decide if it’s the CPU, mobo, power supply or ‘other’. I’d agree that it’s likely one of the above. But lets think of a few other possibilities. have you tried re-seating the CPU fan to motherboard connection? Some mobos won’t boot if they can’t talk to the CPU fan tachyometer (although they should fail with audible warnings). Are there missing/broken blades on the fan that’re impeding its motion? Does your mobo have nice friendly bios sequence lights for diagnosing these kinds of situations (most likely it will be a line of 4 leds that change colour rapidly on boot) I doubt you’d have missed these, but maybe they’re tucked away somewhere. Consult your motherboard docs if these lights are present, they should tell you what’s gone wrong. Voodoo options: I’m not a harware guy but I’ve seen the following used with some success: Plug out the PSU - mobo connection, leave for 1/2 hour and plug it back in and try again (if anyone cares to explain this I’d love to hear it) The hardware guy in work has a habit of smelling the motherboards to detect burnt components, claims it’s far easier than any other method, if a component has burnt out he knows just to unwrap another and start again. ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 10, 2004, 7:20pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/4 "2004-08-10T19:20:50Z")
> [@counsel wolf](#): > > have you tried re-seating the CPU fan to motherboard connection? Some mobos won’t boot if they can’t talk to the CPU fan tachyometer (although they should fail with audible warnings). Yes, actually. I tried that. I even tried it in the second CPU fan power supply. > [@counsel wolf](#): > > Are there missing/broken blades on the fan that’re impeding its motion? No. Everything _looks_ to be in great shape. > [@counsel wolf](#): > > Does your mobo have nice friendly bios sequence lights for diagnosing these kinds of situations (most likely it will be a line of 4 leds that change colour rapidly on boot) I doubt you’d have missed these, but maybe they’re tucked away somewhere. Consult your motherboard docs if these lights are present, they should tell you what’s gone wrong. I just have a green power supply LED and a red LED of indeterminate purpose. I checked the mobo docs- nada > [@counsel wolf](#): > > Voodoo options: It’s all voodoo to me > [@counsel wolf](#): > > Plug out the PSU - mobo connection, leave for 1/2 hour and plug it back in and try again (if anyone cares to explain this I’d love to hear it) This is an easy one to try. I’m on it. > [@counsel wolf](#): > > The hardware guy in work has a habit of smelling the motherboards to detect burnt components, claims it’s far easier than any other method, if a component has burnt out he knows just to unwrap another and start again. I’ve a friend who does the same thing. Unfortunately, he’s currently unavailable for comment. Nothing smells amiss except the stress sweat of yours truly. Thanks **counsel wolf** ---
**Author:** ![badmana](https://avatars.discourse-cdn.com/v4/letter/b/977dab/32.png) [@badmana](https://boards.straightdope.com/u/badmana)\ **Post date:** [August 10, 2004, 8:21pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/5 "2004-08-10T20:21:32Z")
I almost read that all wrong. So you’re getting a blank screen and no beeps? Generally that’s a bad thing. The POST (power on self test) detects the CPU and starts everything up. No CPU = no start. If there’s power to the lights, then there’s power out of the Power Supply. Do the hard drives spin up? Most likely, the CPU went bad when the fans stopped working. My friend once started a computer with no fan/heatsink and it went in seconds (puff of smoke, the whole works). Most likely, the high heat wore down the CPU over the course of a few days and went bad. Pick up another one (cheap, used, whatever) and test it. Otherwise, blank screen errors (with no beeps) could be the mainboard. ---
**Author:** ![Daizy](https://avatars.discourse-cdn.com/v4/letter/d/46a35a/32.png) [@Daizy](https://boards.straightdope.com/u/Daizy)\ **Post date:** [August 10, 2004, 9:22pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/6 "2004-08-10T21:22:28Z")
Could be the processor or the mobo - I don’t believe it’s the PSU… you wouldn’t get any fans running then. Some things will work even with the CPU burned, simply because they work independently of the CPU. As long as there’s power from the PSU… Smelling the mobo is a good idea - burned components emit a smell that’s unmistakable. And usually the board is sprayed with some sealant - lacquer stuff - to insulate and lower brumming sounds from components. And the lacquer stuff stinks when burnt. ---
**Author:** ![Ms2001](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ms2001/32/18463_2.png) [@Ms2001](https://boards.straightdope.com/u/Ms2001)\ **Post date:** [August 10, 2004, 11:01pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/7 "2004-08-10T23:01:44Z")
It could be another component, too. Remove any drives and PCI cards you don’t absolutely need, and double check the connections on anything that’s still plugged in - sometimes a misconnected or misbehaving hard drive can prevent the system from booting. (IDE, at least. I haven’t had the luxury of using SATA.) ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 10, 2004, 11:59pm UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/8 "2004-08-10T23:59:09Z")
> [@badmana](#): > > So you’re getting a blank screen and no beeps? Generally that’s a bad thing. The POST (power on self test) detects the CPU and starts everything up. No CPU = no start. I’m not getting _anything_ at all except my 5volt standby led lights up. > [@badmana](#): > > If there’s power to the lights, then there’s power out of the Power Supply. Do the hard drives spin up? Nothing at all. > [@badmana](#): > > Most likely, the CPU went bad when the fans stopped working. My friend once started a computer with no fan/heatsink and it went in seconds (puff of smoke, the whole works). Most likely, the high heat wore down the CPU over the course of a few days and went bad. I was warned about this by a friend who has killed more CPUs than he will admit to. It’s not the CPU though. I stuck the CPU in this computer and it’s working great. > [@badmana](#): > > Otherwise, blank screen errors (with no beeps) could be the mainboard. This is disturbing ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 11, 2004, 12:13am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/9 "2004-08-11T00:13:08Z")
> [@Daizy](#): > > Could be the processor or the mobo - I don’t believe it’s the PSU… you wouldn’t get any fans running then. Some things will work even with the CPU burned, simply because they work independently of the CPU. As long as there’s power from the PSU… I’m not getting any fans. I tried my PSU in this computer that I’m using right now. It didn’t work. However, putting the good PSU from this computer into that one didn’t make any difference- still nothing. The two PSUs are both ATX and both from the same company and both have these outputs: 350W +3.3v 20A Orange +5v 30A Red +12v 12A Yellow +5vsb 2A Purple -5v 0.5A White -12v 0.8A Blue ps-on Green p.g Grey **Are these outputs enough to establish that the PSUs are interchangable?** If not, what do I look at to figure out if they are interchangable? > [@Daizy](#): > > Smelling the mobo is a good idea - burned components emit a smell that’s unmistakable. And usually the board is sprayed with some sealant - lacquer stuff - to insulate and lower brumming sounds from components. And the lacquer stuff stinks when burnt. I’m not smelling anything but me. ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 11, 2004, 12:18am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/10 "2004-08-11T00:18:02Z")
> [@Mr2001](#): > > It could be another component, too. Remove any drives and PCI cards you don’t absolutely need, and double check the connections on anything that’s still plugged in - sometimes a misconnected or misbehaving hard drive can prevent the system from booting. (IDE, at least. I haven’t had the luxury of using SATA.) I’ve tried to get things going with only the PSU attached to the mobo- nada. I’m really puzzled over the PSU compatability. As far as I know, if they have the same voltages and form, then they should be interchangeable, right? ---
**Author:** ![Ms2001](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ms2001/32/18463_2.png) [@Ms2001](https://boards.straightdope.com/u/Ms2001)\ **Post date:** [August 11, 2004, 12:25am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/11 "2004-08-11T00:25:52Z")
> [@SimonX](#): > > I’m really puzzled over the PSU compatability. As far as I know, if they have the same voltages and form, then they should be interchangeable, right? Yes. In fact, as long as they’re both ATX power supplies, they should be interchangeable–the pinouts are part of the standard–unless you need a Pentium 4 power connector and one of them doesn’t have it. You say you’re getting no fans at all, even the PSU fan. That indicts either the PSU or the motherboard (which tells the PSU when to turn on). You also say that the PSU from the bad computer won’t work in the good computer. That focuses the blame on the PSU. It may have fried the motherboard when it died, which would explain why the good PSU won’t work in the bad computer. ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 11, 2004, 12:50am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/12 "2004-08-11T00:50:20Z")
> [@Mr2001](#): > > Yes. In fact, as long as they’re both ATX power supplies, they should be interchangeable–the pinouts are part of the standard–unless you need a Pentium 4 power connector and one of them doesn’t have it. > > You say you’re getting no fans at all, even the PSU fan. That indicts either the PSU or the motherboard (which tells the PSU when to turn on). You also say that the PSU from the bad computer won’t work in the good computer. That focuses the blame on the PSU. It may have fried the motherboard when it died, which would explain why the good PSU won’t work in the bad computer. I have mixed emotions about this diagnosis. A friend suggested that possibly a loose screw got behind the mobo and shorted something. I’m going to take it all apart and then re-assemble it. ---
**Author:** ![theckhd](https://avatars.discourse-cdn.com/v4/letter/t/cdc98d/32.png) [@theckhd](https://boards.straightdope.com/u/theckhd)\ **Post date:** [August 11, 2004, 1:04am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/13 "2004-08-11T01:04:45Z")
When you run into situations like this that seem to have conflicting evidence, the best bet is to strip 'er down and run on barebones. This is often what tech support will advise you to do: Take everything apart, put the mobo on an insulating surface, and assemble the essential bits outside of the case: Mobo Processor w/fan Video Card Power supply you don’t even need to plug the video card in if you don’t have a monitor that’s easily movable. Try turning the system on (for this you probably need the case nearby so you can plug in the case’s ATX power switch – unless you have a spare switch lying around). If you’re getting absolutely no power (meaning no power on the mobo LED) then you should swap the power supply out for another one and see if that fixes it (sounds like you’ve already done this, but it can’t hurt to try again with the motherboard out of the case, removing the possibility of a short circuit). If the mobo LED comes on, but you don’t get fans/etc. spinning and no bios beep codes, it’s possible that the power supply is putting some voltages out correctly, but not others. You can test this with a multimeter ( **after** disconnecting the ATX power header from the mobo) or with a commercial tester (CompUSA sells them for a ridiculously high price, like $10-20). If you have a Power Supply that’s known to be good, and you get nothing from the mobo, then the next thing to check is the motherboard itself or the ATX power switch (especially since from your description the CPU/fan is fine). I’ve had switches go on me before, at which point the system will not respond at all (i’ve also had ones that would turn on and then off again because of stray grounding). If you can “borrow” the switch from your friend’s case (meaning set up your “barebones kit” next to his case and connect his ATX power switch to your motherboard’s connector), that would tell you whether the switch is at fault. If all of these fails, it’s probably the motherboard at fault (if the video card was the only thing at fault, you should at least get a bios beep code, so we can rule that one out). Call the tech support line of your motherboard manufacturer and explain what you’ve done – often they’ll have you repeat some of the tests, at which point they can issue you an RMA if the mobo is under warranty. If it’s not under warranty, it may be time to order a new motherboard. ---
**Author:** ![Ms2001](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ms2001/32/18463_2.png) [@Ms2001](https://boards.straightdope.com/u/Ms2001)\ **Post date:** [August 11, 2004, 1:17am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/14 "2004-08-11T01:17:36Z")
> [@theckhd](#): > > If the mobo LED comes on, but you don’t get fans/etc. spinning and no bios beep codes, it’s possible that the power supply is putting some voltages out correctly, but not others. You can test this with a multimeter ( **after** disconnecting the ATX power header from the mobo) How does that work? Don’t you need the PSU plugged into the mobo to turn it on? ---
**Author:** ![theckhd](https://avatars.discourse-cdn.com/v4/letter/t/cdc98d/32.png) [@theckhd](https://boards.straightdope.com/u/theckhd)\ **Post date:** [August 11, 2004, 1:43am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/15 "2004-08-11T01:43:55Z")
Sorry if i was unclear with the explanation. You can test **the power supply** by disconnecting it from the motherboard, plugging in the power supply, and testing the voltage of each pin of the ATX power header with a voltmeter. This step does not involve the motherboard at all. When i said > [@theckhd](#): > > You can test this with a multimeter … i was referring to testing the voltage of each pin of the power header to check that all of the voltages were close to spec. The preliminary test (where the mobo LED comes on but no fans power up) would of course require the power supply connected to the motherboard, as you pointed out. 😃 ---
**Author:** ![Ms2001](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ms2001/32/18463_2.png) [@Ms2001](https://boards.straightdope.com/u/Ms2001)\ **Post date:** [August 11, 2004, 1:53am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/16 "2004-08-11T01:53:06Z")
> [@theckhd](#): > > Sorry if i was unclear with the explanation. You can test **the power supply** by disconnecting it from the motherboard, plugging in the power supply, and testing the voltage of each pin of the ATX power header with a voltmeter. Well, my understanding is that an ATX power supply is turned on by the motherboard, so a multimeter will just show a bunch of zeros if it’s not connected. According to [this review of a PSU tester](http://www.dansdata.com/quickshot018.htm), you can short the green wire to ground with a paperclip to make it turn on, if you don’t have a mobo or tester attached. ---
**Author:** ![theckhd](https://avatars.discourse-cdn.com/v4/letter/t/cdc98d/32.png) [@theckhd](https://boards.straightdope.com/u/theckhd)\ **Post date:** [August 11, 2004, 2:19am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/17 "2004-08-11T02:19:48Z")
> [@Mr2001](#): > > According to [this review of a PSU tester](http://www.dansdata.com/quickshot018.htm), you can short the green wire to ground with a paperclip to make it turn on, if you don’t have a mobo or tester attached. Ah, yes. I forgot to mention that, probably because I haven’t done this in a long while, so I’m a bit rusty on the details. I just knew that it could be done with the proper pin-out (which i figured someone would come along and provide 😃 ) and equipment. Thanks for catching that important oversight. ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 11, 2004, 2:55am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/18 "2004-08-11T02:55:09Z")
Thank you for your detailed and helpful sounding response. Now would be the time for me to confess my rampant ignorance of things computer, because I’m fixing to ask you a bunch of basic questions. I hope you have the time and inclination to check back and answer them. > [@theckhd](#): > > Take everything apart, put the mobo on an insulating surface, Would wood work? Would a particle board folding table with a faux wood veneer be acceptable? If not, what would you suggest? > [@theckhd](#): > > Try turning the system on (for this you probably need the case nearby so you can plug in the case’s ATX power switch – unless you have a spare switch lying around). By ATX power switch you’re referring to the power supply box, right? Can I just pull it out of the case and lay it out together on the same insulating surface as the other components? > [@theckhd](#): > > You can test this with a multimeter ( **after** disconnecting the ATX power header from the mobo) or with a commercial tester (CompUSA sells them for a ridiculously high price, like $10-20). This sounds really cool. I’d think I was something else if I knew how to do this. Is it very complicated? Or can I perform the procedure with only a modicum of training and planning? Where can I find the necesary info to engage in this sort of adult activity? > [@theckhd](#): > > If you have a Power Supply that’s known to be good, and you get nothing from the mobo, then the next thing to check is the motherboard itself or the ATX power switch (especially since from your description the CPU/fan is fine). Oh, so the ATX power switch is not the same as the power supply box. **What’s an ATX power switch?** > [@theckhd](#): > > If all of these fails, it’s probably the motherboard at fault (if the video card was the only thing at fault, you should at least get a bios beep code, so we can rule that one out). Call the tech support line of your motherboard manufacturer and explain what you’ve done – often they’ll have you repeat some of the tests, at which point they can issue you an RMA if the mobo is under warranty. If it’s not under warranty, it may be time to order a new motherboard. It may b under warranty, I’ll have to check. It’s not that old \<6mos. **What’s an RMA?** ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 11, 2004, 2:57am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/19 "2004-08-11T02:57:06Z")
> [@theckhd](#): > > You can test **the power supply** by disconnecting it from the motherboard, plugging in the power supply, and testing the voltage of each pin of the ATX power header with a voltmeter. 😃 Is a voltmeter the same thing as a multimeter? ---
**Author:** ![PatriotX](https://avatars.discourse-cdn.com/v4/letter/p/c4cdca/32.png) [@PatriotX](https://boards.straightdope.com/u/PatriotX)\ **Post date:** [August 11, 2004, 3:04am UTC](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231/20 "2004-08-11T03:04:06Z")
> [@Mr2001](#): > > According to [this review of a PSU tester](http://www.dansdata.com/quickshot018.htm), you can short the green wire to ground with a paperclip to make it turn on, if you don’t have a mobo or tester attached. Interesting and somewhat educational. I don’t know enough to get the full benefit. Thank you. [Next page](https://boards.straightdope.com/t/when-cpus-die-do-the-fans-still-come-on/259231.md?page=2)