Why is my imac shutting down by itself?

Nope. I've done nothing to this computer.

This is happening to me too. The first time it did this and came back on was when I unplugged it for about two days. That was about a month ago. Then, about 10 days ago, it happened again. I did the SMC reset, unplugging it and pushing the power button for ten seconds, plugging it back in. I thought it was a overheating problem so I keep my room cool and even have a fan blowing behind it.

Is this just a fault in the production, or is it something I'm doing wrong? It's pretty frustrating, and being in Germany without a car, the thought of lugging this thing down the S-Bahn downtown doesn't seem pleasant. I hope to be able to call Apple and see what is going on, because I atleast have Applecare on the thing....

I too am having this symptom. My computer shuts down randomly. I can't put my finger on the problem. I bought my 27 inch iMac in June of 2012. Seemed ok for awhile...then I upgraded to Mountain Lion and issues started to surface...not sure if the issue is connected to Mountain Lion. But boy is this frustrating. I looked so forward to getting this new iMac and this issue is killing me....

I will be calling Apple tonight after work but I just wanted to make clear that it happened to me a couple of times before I upgraded to Mountain Lion. I just upgraded to Mountain Lion on Sunday and it happened to me on Monday night, so it doesn't seem to be a problem exclusive to the new OS.

I'm thinking this might be just a fault in the production line? The only resolution I have seen through many forums I've searched on was to reset the SMC, and although this worked for me once, it did not work this time around. I'm not sure if there is a thread that has an actual answer; if there is one, I have yet to happen upon it.

Also, I have not upgraded any RAM or anything; the computer is stock in all aspects.

I called Apple yesterday...they tried to walk me through a process of rebooting with holding down a few keys...something that has been discussed here before. I could not make the process work with bluetooth keyboard or trackpad. And besides that does not seem to remedy problem for long.

I have a stock machine as well...last night i plugged my iPhone in to see if the charging light on it would still be shown when my monitor is blank...it was.

Apple over the phone I dont think can help...probably have to drive a frigging hour to have Genius tell me they cant help either.

I have been experiencing this same random shut down! It started yesterday in the middle of taking a test on line!! Thankfully I was able to get back to the test and finish it after starting my iMac again. It happened again a little while ago too. =/ My computer was plugged into a power strip with my printer, which does not lose power when my computer does. I think Apple needs to figure out what is going on with all these iMac's, I would hate to lose trust in this so far quality product.

Just found this on another site...trying it now.

A couple of things. First use Disk Utility to repair permissions.

Then download and install the free iStat Pro widget:
http://www.islayer.com/apps/istatpro/

After it is installed, check your temperatures. On my 2 year old iMac, the CPU was running@140F and the hard drive 125F; too hot in my opinion. I downloaded and installed the free smcFanControl:
http://www.eidac.de/

and bumped up the internal fan speeds by 4-500rpm. Now my CPU runs at 115F and hard drive 105F.

I run the fan control on my laptop too. Same positive results in temps with just a fractioal loss of battery life.

Best of luck.

Source(s):

http://www.eidac.de/

This is a defective power supply.

this happened in my imac 2008. I was using it and then all of a sudden the computer would turn off. restarting with the power button and then would shutdown again.

Take it to Apple and they will replace the power supply. If it is in warrenty it will be free. failing that if it is out of warrenty it will cost around £80 or $126 approx.

Let me know how you get on.

Same problem here. I have a 27" 2011 iMac running 10.6.8 with 16GB. In fact I can now have it shut down at will by itself. As soon as I work on very large data sets in Photoshop within 30s-120s it suddenly shuts down. Never added hardware to it (memory etc) or made mods, I am using it as it came from Apple.

For instance, I am now unable to work on a 2GB file in Photoshop (lots of layers) because as soon as I open it after a few seconds the iMac shuts down ---without any warning of course --- by itself.

Hello,

New mac owner here literally on the 29th of sept i bought my first mac imac that is and here we are on wednesday and it shuts down randomly and the browser goes out unexplainably and it would not let me in anything like disk utility or anything like that. So seeing i bought it at best buy they took it back no questions asked and gave me a new one. any luck fixing your issues?

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.