mikestan56

Q: iMac shuts down randomly for no apparent reason

In late August of this year, after the plug came out of the back of my iMac when I was attempting to adjust the angle of the screen while the computer was on, the computer began shutting off for no reason while on. I took the iMac to the Apple Store in El Paseo, in Palm Desert. They replaced the 3 TB fusion hard drive saying that there had been a history of problems with this. I received the iMac back and shortly thereafter began to shut down once again for no reason. I brought it back to the Apple Store and they deleted some software and removed some software that were automatically engaging at startup. The iMac began to shut down for no reason once again. I brought it back in and they ran a full diagnostic test and could not find any problems. It worked for a while and then began to shut down for no reason again. I brought it back in to the Apple Store in this time I was advised that there are going to run more extensive hardware testing, running it through longer cycles etc. They could not find the problem and suggested that it was likely an effect of peripherals I had connected to the iMac. I disconnected all the peripherals from the iMac and began testing them one by one over time. The iMac still shut down for no reason at random times. I ran the computer with no peripherals connected at all, and it still shut down for no reason. I have run Apple diagnostics and it found no trouble. I have run malware detection software and it found some things that were then deleted. I did this very early before I even brought to the Apple Store. I ran antivirus scans, full scans on the iMac and it found no infection. I also ran fan control software to see if the iMac was overheating and even with the fans at the full rpm setting, the iMac still shut down at random times. I used a rocket air blower that I used to remove dust from my camera lens to try to blow through the back vent and the bottom screen vents to free that of any buildup of dust, and it still shut down for no reason. What could possibly be the issue with this machine after all the testing and interventions that have been done?


Late 2012 iMac - Quad Core Core i7- 16GB, 3TB Fusion Drive

iMac, OS X Yosemite (10.10.4), Mac (27-inch, Late 2012) 3.4 GHz

Posted on Nov 3, 2015 3:55 PM

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Q: iMac shuts down randomly for no apparent reason

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  • by John Galt,Helpful

    John Galt John Galt Nov 4, 2015 10:57 AM in response to mikestan56
    Level 8 (49,085 points)
    Mac OS X
    Nov 4, 2015 10:57 AM in response to mikestan56

    What could possibly be the issue with this machine after all the testing and interventions that have been done?


    A hardware fault. Don't waste your time with any "malware detection software", "antivirus scans", or any similarly categorized garbage.

     

    Open Console - it's in your Mac's Utilities folder. To determine the shutdown cause, examine system.log for "shutdown cause". The log entry will be "Previous shutdown cause:" followed by a numeric code. What is that code?

  • by mikestan56,

    mikestan56 mikestan56 Nov 4, 2015 11:03 AM in response to John Galt
    Level 1 (4 points)
    Nov 4, 2015 11:03 AM in response to John Galt

    I investigated this last night after work and because it has not shut down the last two days, the shutdown code was not in the system log. I will be working on it more intensively this week and I will provide the code as soon as it shuts down again. Is there any way to access system logs further back in time. It only showed the log for yesterday and I could not see how to go back further than that. Thank you for your help, you have been more help in one paragraph than four trips to the Apple Geniuses! - Mike

  • by John Galt,

    John Galt John Galt Nov 4, 2015 12:41 PM in response to mikestan56
    Level 8 (49,085 points)
    Mac OS X
    Nov 4, 2015 12:41 PM in response to mikestan56

    Is there any way to access system logs further back in time.


    Yes. Archived logs are named system.log.0.gz and so forth. They can be opened and examined the same way as system.log, which is normally archived daily at midnight system time.

  • by mikestan56,

    mikestan56 mikestan56 Nov 4, 2015 8:48 PM in response to John Galt
    Level 1 (4 points)
    Nov 4, 2015 8:48 PM in response to John Galt

    previous shutdown cause 0

  • by mikestan56,

    mikestan56 mikestan56 Nov 4, 2015 8:51 PM in response to John Galt
    Level 1 (4 points)
    Nov 4, 2015 8:51 PM in response to John Galt

    Here are three:

    Nov  3 21:10:26 localhost kernel[0]: Previous shutdown cause: 5

    Nov  4 17:36:11 localhost kernel[0]: Previous shutdown cause: 5

    Nov  4 20:38:29 localhost kernel[0]: Previous shutdown cause: 0

  • by mikestan56,

    mikestan56 mikestan56 Nov 4, 2015 9:00 PM in response to John Galt
    Level 1 (4 points)
    Nov 4, 2015 9:00 PM in response to John Galt

    John , this is what console recorded just at shutdown and then reboot

     

    Nov  4 20:36:25 Macintosh-2 networkd[189]: -[NETProxyLookup pacLookupComplete:proxies:error:] PAC evaluation error: Error Domain=kCFErrorDomainCFNetwork Code=2 "(null)" UserInfo={kCFGetAddrInfoFailureKey=8}

    Nov  4 20:36:36 Macintosh-2 networkd[189]: -[NETProxyLookup pacLookupComplete:proxies:error:] PAC evaluation error: Error Domain=kCFErrorDomainCFNetwork Code=2 "(null)" UserInfo={kCFGetAddrInfoFailureKey=8}

    Nov  4 20:38:22 localhost bootlog[0]: BOOT_TIME 1446698302 0

     

    Is this useful at all?

  • by John Galt,

    John Galt John Galt Nov 4, 2015 9:08 PM in response to mikestan56
    Level 8 (49,085 points)
    Mac OS X
    Nov 4, 2015 9:08 PM in response to mikestan56

    Previous shutdown cause: 0


    That code will appear if the power to your Mac is suddenly interrupted. You can confirm that yourself by disconnecting its power cord from the wall receptacle while the Mac is operating.

     

    The causes can be due to any one of the following

     

    1. a defective power cord, which might not be evident by inspecting its exterior
    2. an intermittent connection between the power cord and the Mac's power receptacle
    3. a faulty (household / office) power receptacle
    4. a faulty power strip / surge suppressor
    5. a defective Mac power supply
    6. a crack or similar break in the Mac's circuit board

     

    The most likely cause given the history you reported is #1. You can substitute any standard electronic device power cord that fits.

  • by mikestan56,

    mikestan56 mikestan56 Nov 5, 2015 6:03 AM in response to John Galt
    Level 1 (4 points)
    Nov 5, 2015 6:03 AM in response to John Galt

    More shutdowns

     

    Nov  3 21:10:26 localhost kernel[0]: Previous shutdown cause: 5

    Nov  4 17:36:11 localhost kernel[0]: Previous shutdown cause: 5

    Nov  4 20:38:29 localhost kernel[0]: Previous shutdown cause: 0

    Nov  4 21:35:02 localhost kernel[0]: Previous shutdown cause: 0

    Nov  5 05:51:54 localhost kernel[0]: Previous shutdown cause: 0

     

    They gave me a new power cord after my last visit. Maybe its one of the other reasons you listed...

  • by John Galt,

    John Galt John Galt Nov 5, 2015 8:18 AM in response to mikestan56
    Level 8 (49,085 points)
    Mac OS X
    Nov 5, 2015 8:18 AM in response to mikestan56

    Power interruption is causing the random shutdowns. Apple's support "geniuses" should have been able to determine that within minutes of accepting it for repair the first time, much less three or four return visits.

  • by mikestan56,

    mikestan56 mikestan56 Nov 6, 2015 8:58 AM in response to John Galt
    Level 1 (4 points)
    Nov 6, 2015 8:58 AM in response to John Galt

    John, thank you for all of your feedback on this issue.I have replaced the power cord, switched to a different surge protector, moved the computer so that is plugged into a different outlet, and I'm still getting shut down. At some of my many trips to the Apple Store to get service on the iMac, they apparently did extensive hardware testing to see if there was a problem. I also ran Apple diagnostics three times to see if it could detect any hardware issues and it came back with no trouble found. If it is a defective Mac power supply or a crack in Mac circuit board wouldn't that have been identified in a hardware assessment? It is important that I get this computer fixed as it is business essential. What do you suggest I do further to address the hardware issue? I'm wondering if I should take it to a different computer repair business that is certified in working on Apple equipment. I appreciate your thoughts on this and thank you.-Mike

  • by John Galt,

    John Galt John Galt Nov 6, 2015 10:58 AM in response to mikestan56
    Level 8 (49,085 points)
    Mac OS X
    Nov 6, 2015 10:58 AM in response to mikestan56

    If it is a defective Mac power supply or a crack in Mac circuit board wouldn't that have been identified in a hardware assessment?


    No, other than to identify the cause of the shutdown being a power interruption. The power supply itself might be defective, but all the logic board (the SMC to be specific) is able to determine and report is an unexpected loss of power. The same shutdown cause would occur if there were a break in any circuit between the iMac's power source and the logic board. The break might be microscopic and difficult to find. It might also change during physical movement or thermal expansion, which can be induced by bending or stressing various connectors for example.

     

    It could also be brought on by a short anywhere in the power supply, perhaps an intermittent one which could have been caused by an object becoming dislodged from the back of the iMac. 27 inch iMacs such as yours have been known to have that bracket fail internally. It's possible an internal fastener became loose and found itself conducting an electrical path where one ought not to be. Loose things or otherwise conductive debris inside a computer will cause random failures that can be difficult to find.

     

    As you see there can be quite a few potential causes. A problem such as that might require board-level repair, which has become a lost art. Most computer repair shops would prefer to substitute components, such as the power supply (which is not expensive) or the logic board (which is), observe the results, and stick you with the bill.

  • by mikestan56,

    mikestan56 mikestan56 Nov 6, 2015 2:23 PM in response to John Galt
    Level 1 (4 points)
    Nov 6, 2015 2:23 PM in response to John Galt

    Would you advise that I bring the iMac back to Apple with the information you have provided, and ask that they either replace the power supply or the logic board?

  • by westom,

    westom westom Nov 6, 2015 6:05 PM in response to mikestan56
    Level 1 (4 points)
    Nov 6, 2015 6:05 PM in response to mikestan56

    At what point was someone going to discuss or target the only item that powers on or off a machine?  A power controller takes various inputs to decide when to power cycle.  It even determines when the CPU is permitted to execute.  One controller input is the front panel power button.  Controller even acts differently depending on how long that button is pressed.  Another input comes from the OS - when the OS asks for a power off.  If the OS asks for a sudden power off, then the OS also says so in logs.  Another input is monitor circuits in the power supply.  If those demand a power off, then the OS and its logs know nothing about it.

     

    How to find an informed tech?  He knows about this hardware and how to measure it.

     

    What diagnostics did Apple techs run?  A processor called the power controller has no hardware diagnostics.  CPU cannot monitor or obtain status information.  CPU is simply told by this processor  when to and not to execute.  What diagnostic could Apple (software) techs run to diagnosis a part of hardware that software cannot see?  IOW get an informed tech.

     

    Power cord and surge protector clearly are irrelevant.  Your Apple must work just fine even when incandescent bulbs dim to 40% intensity.  How did a power cord or power strip cause voltage to drop that much - without also getting hot and dangerous?

     

    Your problem clearly requires someone who knows hardware. Who does not blame things without any reason to.  And has simple layman tools to view what that hardware is doing.  Again, find a tech who knows how electricity works. That is a first problem.  Power off solution comes later.

  • by John Galt,

    John Galt John Galt Nov 6, 2015 8:04 PM in response to mikestan56
    Level 8 (49,085 points)
    Mac OS X
    Nov 6, 2015 8:04 PM in response to mikestan56

    I recommend you bring it back to them and tell them it's not fixed yet. Whether to replace the logic board, power supply, or both is a question of who's paying the bill.

     

    The problem you describe is identical to, and indistinguishable from, a power interruption. That power interruption can be internal to your Mac or external to it. That's all anyone knows. I described the potential causes, and they are not limited to the power supply or the logic board. It would be unfortunate to replace one or the other just to find that your building power is unreliable. You could eliminate that possibility by using a UPS from a reputable manufacturer.

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