Why is my imac shutting down by itself?

This was happening to me also, and I have finally figured it out. First the good news: There is nothing wrong with your computer. There is no bad news. This is happening because of your Security and Privacy preferences in the System Preferences. So,

1. Open System Preferences

2. Open Security and Privacy control panel

3. Click on the General tab

4. Click on the Advance button at the bottom. If it is grayed out, you need to click on the lock icon and enter your login password.

5. Uncheck the first box labeled 'Log out after ?? minutes of inactivity'

6. Press OK

7. Close the control panel.

You are good to go. What is happening is the system is trying tp log out the current user beacuse it is inactive for ?? minutes. This must be the default used after last system upgrade (bad choice Apple!).

We had the same problem on one of our work computers. It was a newer Imac I5 Here is what I did.

1) Check your Console (located in Applications/Utilities) if you know roughly when it shut down, then odds are it's in the log, if not, it could indeed be a hardware problem, in our case it appeared to be software related since the Imac checked out ok at Apple Store.

2) Notice what apps are running at the time of the shutdown, and again check the Console

3) In this case, we had a program we had removed that was still in the LaunchDeamons, in fact I found many startup items that where either dead or outdated. If you know Macs, it is very difficult cleaning out residual files left by a deleted Application. So far under multiple tests, the computer is no longer turning off the screen by itself. Just do not delete what you are not sure of, it can mess up your computer worse. In our case it was simple since the file name corresponded to a App no longer on the machine which was obvious.

4) After I deleted all the old files, I ran Disk Utility Repair tool, and so far so good. (it would shut off intermittenly anywhere between a few minutes and an hour), after 6 hours it has stayed on just fine under multiple tests.

5) I also did some research on another rogue core function that sends the computer into a hibernate mode, the Imac was showing a previous shutdown code of 5 which seems to be the same as when you shut the lid on a macbook, that is the next thing I am checking on if the problem re-occurs again.

I've been having the same problem for a couple of years now. 27-inch iMac running 10.6.8. I added a hard drive, though, through OWC, so figured I had no recourse with Apple. Mostly it shuts down when I'm running Facebook, especially if I scroll. Can't scroll! No scrolling! Or if I try to work on a large Photoshop file. Very unsettling, frustrating. Did you ever find any kind of resolution?

You don't say how much RAM you have installed or if you added any aftermarket RAM. It might be intermitent RAM problem. This could be caused by overheating if the fan or heatsink is full of dust of if the fan bearings are getting a bit "sticky". It may also be, since you are running 10.6, which is a bit older, that you are maxing out the RAM and forcing a lot of disk activity to act, sort of, like additional RAM. Use Activity Monitor to see how your page ins and outs are looking.

You may want to download and install the iStat Pro widget to monitor RAM use, several internal tempratures and a number of other things. You can get it at:

https://www.apple.com/downloads/dashboard/status/istatpro.html

Here is a bit of info on Activity Monitor:

https://support.apple.com/kb/HT1342

I have 16 megs of RAM, no aftermarket RAM, a dedicated SSD for the system and applications, a 2-terabyte drive for internal storage. No overheating is occurring as far as I can tell. The shut-downs happen mostly on facebook for some reason, and always when nothing else is running or open. They happen unpredictably but I can almost guarantee that they will happen if I scroll on facebook or click through too many times on facebook. Often within moments of opening facebook and starting up the computer.

Also happens when I view certain weather maps and try to animate them. And can occasionally occur if I try to open a large photoshop file.

16 MEGs??? Judging from the way the rest of the computer is stocked, I think you mean 16 Gigs.😉

Does the Facebook shutdown occur with any browser (Firefox, Chrome, Safari, other?) or just one particular browser? Same for weather maps. The only thing that seems to indicate it is not a browser issue is that you also get a shutdown, but only ocassionaly when openig a large Photoshop file.

Also, just curious, with the high level of resources, you have invested in the computer, is there some particular reason you are staying at 10.6.8? I wonder if the problem would happen with 10.8. That is only a thought and not a suggesdtion to go to Mt. Lion. Are the other Apple updates (non OS) all current for your system? I mean things like firmware updates, Java updates, security updates, etc.?

Have you tried eliminating all the plugins and extensions for your browser(s)? Just wondering if there is some conflict taking place. Also, is your browser fully up-to-date?

Does the shutdown happen when you are running a bare bones configuration? I mean with no printers, connected, no scanners, USB flash drives, or anything other than just a wired keyboard and wired mouse connected to your USB, Firewire or video ports. Reason I ask is if there are a things connected is I wonder if they might be putting an additional load on the power supply to where, when it is called to really pump out the power for the more demanding applications the power supply kind of chokes out.

Yikes, yes, thanks, I meant Gigs.

It happens ever so slightly more with Firefox than with Chrome. But it happens in both. I don't use Safari if I can help it.

I have a Wacom tablet and an Epson 10,000 scanner attached via USB. And a little HP deskjet printer. I have tried unplugging them... makes no difference.

I am up to date on everything... just haven't wanted to migrate to 10.8 because this is my home computer, which I use in my home studio, and it has to be the same system as I use at work on my PowerMac. Everything is working so well on the tower that I don't want to update just now. If it ain't broke, etc. But I do like to take work home and work on it there at times and lately the iMac just won't do it. Used to, at first. Always been shutting down with facebook, but the shutdowns with photoshop are recent.

Come to think of it, I haven't tried unplugging the scanner. Will try that.

I have not tried working with a wired mouse and keyboard! Great suggestion. I will try that in the next few days if I can and report back.

Ever find a solution for this? I am being affected by the same thing on a mid-2011 21.5" iMac. The one difference with mine though is that when it shuts down, I can't turn it back on unless I unplug the power from it for several hours, then plug it back in.

Nope, no solution! I tried everything. The problem seems to come and go with facebook. It isn't shutting down as much with facebook as it used to. Now the major problem is that I can't open a large photoshop file. Down it goes, immediately. Nothing seems to make any difference except perhaps the phases of the moon. 🙂 I am, however, knock wood, able to restart immediately. You have my sympathy.

dhumes0524,

If it takes many hours to reset itself, it might be a power supply that has a problem and is dying due to overheating. Normally just unplugging the computer for five minutes or so is adequate to reset the SMC. That is the micro program that controls a number of things during the boot up process. It may also be a RAM stick overheating but I would not think that it would not take several hours to cool down.

kenspeckle,

Have you tried pulling out one pair of the RAM to see if the problem changes. If you have four 4 Gig sticks, try pulling out he top pair and replace it with the bottom pair. Then try the replacing the pair that is in with the pair you pulled out. It just might be possible that you have an intermitent RAM problem. Your system should run with 8 Gig installed but it might be a bit slower.

Also does it still lockup when you are running in Safe Mode (hold shift key when powering up)? Also is the activity monitor showing anything running that you do not expect pr even know about?

Well, I left it unplugged overnight and tried it this morning when I came in...I get the chime, grey screen, then it dies before the Apple logo shows. So appears no amount of unplugging is causing it to boot. I agree that it is looking like the power supply. I am going to setup a genius bar appointment to confirm and get an estimate on cost to replace power supply. I had 4 sticks of RAM in it, the 2 original, plus 2 extra ADATA chips. I have tried both sets independently with no difference in the operation of the machine.

I have been reading all these posts and I must say I have been jumping up and down in frustration!

Your problem which you are saying is definitely a power problem and the power pack in your iMac needs to be taken to the apple store and a replacement fitted. If you like in the uk that will cost you around £80. Or $120 or there abouts.

Please book an appointment at the Genius Bar.

Well, I wen't over the weekend and the Genius Bar definitely thinks the problem is a power supply failure. They have taken it to replace the power supply, at a cost of $106, which is far less than I expected, and hopefully I get it back in the next 24-48 and the problem is gone. Will update soon.