iMac switching itself off

Discussion in 'iMac' started by gazareth, Aug 14, 2013.

  1. gazareth macrumors newbie

    Jan 22, 2009

    I have a mid-2011 27" iMac. Two days ago I was streaming video while working on my other computer. The mac switched itself off and then would not turn back on. I called up AppleCare, who advised me to take it in for a service.

    Meanwhile, I did some more googling and came across SMC resetting. I did this and hey presto, the mac turned back on. However, after about an hour of not particularly intensive stuff (copying files from internal HD to external HD, streaming music), it happened again. Once more, an SMC reset was necessary. This time, it switched off again during boot-up.

    I left it for half an hour, then booted in safe mode. I installed SMC fan control and Temperature Monitor. The mac stayed up (and stable, as far as I could tell) in regular mode (i.e. not safe mode) for around 24 hours. Today, I decided to do some stress-testing. I shut off SMC fan control and ran this:

    I watched the temperatures closely while this was happening. CPU activity was getting up to ~96% but none of the temperatures were particularly high. HDD was around 44deg C - the highest was the secondary heatsink which had just hit around 62deg C when the mac switched itself off again. I did an SMC reset (and a PRAM reset just for good measure) this time but it turned itself off while restarting again.

    All of this seems to point to overheating - but none of the temps seem particularly high. Does anyone have any ideas, or am I resigned to taking this in for a service? I'm still within my AC period, but I'd rather not be without my system for a week.
  2. WilliamG macrumors G3

    Mar 29, 2008
    If your system turned itself off while restarting, that doesn't sound like overheating to me...

    I think your best bet is to let Apple deal with it, despite the downtime.
  3. gazareth thread starter macrumors newbie

    Jan 22, 2009
    My logic is that it's getting hot, switching off and then when I try to restart straightaway, it hasn't cooled down yet so it switches off again.
  4. MichaelDT macrumors regular

    Aug 18, 2012
    Friend of mine had the same problem, kept getting worse and worse i.e. shorter intervals. Turned out to be a bad power supply.
  5. WilliamG macrumors G3

    Mar 29, 2008
    There's pretty much no chance of that being the case. The reboot process is going to be a much cooler process than the whatever you were doing before it. I agree a power supply issue seems more likely. Either way, you ---> Apple. :(
  6. dr.oid, Aug 15, 2013
    Last edited: Aug 15, 2013

    dr.oid macrumors newbie


    Aug 11, 2013
    iMac switch off -›use always a good surge protector

    avoid using your iMac in temperatures higher than 30℃, streaming a flash video content in a high temperature room using wifi can cause overheating especially when the heat sinks and the fans are dusty
    in this case the wifi temperature often exceeds 70℃.
    the iMac should have enough free space around it to cool effectively.

    the chance is high to have a faulty power supply.

    although the smc reset can solve firmware and software related problems it cannot repair a hardware failure..

    a time will come when you won't be able to power up your iMac and this failure may cause other damages in your machine.

    there is a chance of a faulty capacitor or something ridiculously cheap and tiny part gone wrong in the power supply

    i think the service will offer you a power supply change

    it would be better for everybody to REPAIR these kind of failures and not to change whole electronic boards and parts to throw the faulty parts away because of a 10cent capacitor or whatever..
  7. gazareth thread starter macrumors newbie

    Jan 22, 2009
    Update for the record: the power circuits on the logic board were failing. Apple replaced the logic board and all is now well again.
  8. talmy macrumors 601


    Oct 26, 2009
    This gets me as well. Unfortunately, unfortunately technicians that have the knowledge to diagnose and repair at this level are expensive. It's cheaper to swap boards and either recycle or refurbish in a central facility the defective board.
  9. volcomfever macrumors newbie

    Apr 11, 2012
    go to console, type in shutdown causes, post the findings here. That will tell you exactly why it's shutting down.
  10. martinwinlow macrumors newbie

    Jun 7, 2010
    I've been having this issue for a few weeks now. Realistically, there are going to be many possible causes of a Mac shutting down by itself.

    People have mentioned several possibilities and I have tried many of the obvious possible solutions including hardware tests, disconnecting all peripherals, removing all my startup programs, installing a temperature monitoring app, powering the iMac from a plain socket rather than through a UPS, disconnecting the wired router connection etc etc and nothing is helping... ****except**** that my iMac is completely stable in Safe Mode.

    Surely this indicates a software problem? Can someone confirm this?

    Is there some app or test I can do to rule out a logic board or PSU problem? Again, if my iMac works fine in Safe Mode surely this would eliminate logic board or PSU issues? Not to mention RAM and overheating ones?

    Regards, Martin Winlow.
  11. JohnPi314 macrumors member


    Jun 27, 2014
    A quick search shows this is happening to others. One thing that was mentioned is faulty memory; so try replacing the original Apple memory and see if the problem persists. If you don't have the original Apple memory, try removing a stick and see if the problem goes away.
  12. martinwinlow macrumors newbie

    Jun 7, 2010
    Hi John,

    Actually I changed the original 4Mb of ram for 8 about 2 months back. It worked fine for 6 weeks before this problem appeared. I have also tried replacing the original RAM and it made no difference.

    Also, if RAM IS THE ISSUE, why is safe mode so stable with no shutdowns or anything at all in the way of problems?

    I am aware the graphics accelerator is not active when in safe mode. I have read in other threads of a problem with the graphics card overheating and causing artefacts and then shutdowns. Never had artefacts tho...

    Thanks for your suggestions. MW
  13. Lee Torian macrumors newbie

    Dec 2, 2014
    iMac is restarting in the middle of doing very little


    06:58 Kernel: Previous Shutdown Cause: 3

    08:59 Kernel: Previous Shutdown Cause: -128

    09:56 Kernel: Previous Shutdown Cause: -6

    How to interpret this?
  14. talmy macrumors 601


    Oct 26, 2009

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