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My iMac keeps restarting every time I turn it on. Sometimes it doesn’t get to the desktop before restarting and other times it restarts while using it. I have already replaced the power supply with the same result. I also have tried unplugging both the HDD and the SSD and it still was restarting. I have tried switching out the RAM from another iMac and it still restarts. When it restarts, it randomly shuts off and then boots again, sometimes over and over. I have already taken it to an authorized Apple tech, and they told me they could never get it to reboot… the closet Apple Store is over 3 hours away so I am trying to troubleshoot this myself. Any ideas on what I can try next? Thanks for your help!

I think you started down the right path as being a power issue. Given the fact the Apple tech couldn’t get it to fail when he had it I suspect your issue is within your homes power not your system. First you’ll need to get a cheap AC Outlet Tester to check your outlet to see if the the outlet is correctly wired. You could also have a grounding issue if you don’t have a three prong outlet. This is were you will need to call in your local electrician to fix the outlet. Even still you do need to make sure the outlets wiring is correct (don’t cheat here, you need all three wires). Following the wiring back to your fuse/breaker panel make sure you have a 20Amp circuit for 120volts and ideally you should have a dedicated line for your computer and your peripheral gear. Make sure the buildings Ground circuit is in good shape. Often a grounding rod stuck into the ground or strapped to the metal water line feeding the building. Over time the wire and/or the connection point corrode (green stuff) and need to be cleaned. The rod can also have degraded and may need to be replaced or positioned in a better location to be effective. Many people put electricians grease on the exposed surfaces of the connections after wiring it up to help prevent corrosion. So we went as far as the fuse/breaker panel and we still have problems. You may need to get a UPS to stabilize the power as your power provider maybe having problems giving you reliable power. Many of the better units have power meters which can show you how good/bad your power is. You could ask your utility provider to test your power: Have them put a power monitor in your house for a good week or so. They may need to service your homes power feed or replace a transformer that feeds your neighborhood.

My iMac (27-inch, Mid 2011) had the same rebooting problem. It would reboot several times after Sleeping or if powered on. However, after a few self initiated reboots the iMac would be stable as long as I kept it on. I tried installing a different brand (WD) new SSD and worked upwards from Yosemite to High Sierra. I also swapped various certified sodimm memories and I even replaced the logic board. All with no positive effect - the rebooting fault persisted. Further, video testing and memory testing apps always proved the hardware was good. Finally, I put my original logic board back in and this time I physically removed the WiFi card and InfraRed Card. Previously, I has disabled the WiFi via the Operating System without result. With the WiFi and infrared cards removed, my system is now stable. I suspect the fault is with the WiFi card as I recall having a similar problem with the wifi card in my old 17” 2006 model macbook pro. I had to remove its wifi card to solve it. Currently using the ethernet LAN cable.

I encountered this same issue a month ago with my 27” Mid 2011 3.4gig I7 iMac 6970M 2Gig video card, 32gig ram, with a split FusionDrive( 2TB HDD/250 Samsung Evo850 SSD ). I tried everything that others have tried( PRAM/SMC reset, Disk Utility from Recovery, erasing the SSD drive, reinstalling the OS, RAM modules, Power Supply from iFixit was ordered in hopes that was it. I upgraded the RAM from 20gig to 32, that didn’t help. I replaced HDD..still didn’t fix it. I replaced the power supply with the one I bought from iFixit, but that didn’t work either. For some strange reason, this only made my fans spin at full blast, and my computer would go to sleep every 27 seconds on the nose, very annoying, but the restarts were more intermittent and not as repetitive. Then I replaced the SDD drive, but it still didn’t solve the problem..which got me thinking a video card re-bake may help. Or maybe something to do with the power button, because when the computer shut down to restart, it would make the same sound it makes when the power goes out. I took my iMac apart, and as I was removing the plugs for the power button and the IR sensor, I noticed that they came out easier than I remember..maybe that was causing some kind of feedback error which caused the computer to shut down and restart. I removed the video card, baked it( 400F/200C for 8 minutes-middle rack ). I replaced the thermal paste, on the i7 processor as well, put it all back together making sure that all the plugs were seated correctly, and now it runs like new again..it’s been 24 hours with zero issues. What a headache this has been..I love this machine and don’t need a new one. The video card problem is because of the crappy heat sink tolerances..there’s a 3-4mm gap between the heatsink and all the chips on the card..thermal paste has to be blobbed on to the chips for it to make contact with the heatsink, which defeats the purpose. I used some thin aluminum plates, cut to the same size as the protrusions on the heatsink, and used thermal paste on both sides of the aluminum plate between the chips and the heatsink, so now most of the chips are making contact with the aluminum for better heat transfer. Temps are good, and I expect the video card will last longer before the next burn. I hope this helps someone else out and maybe saves them a buck or two, but most of all saves them some TIME!

On the iMac (21.5-inch, Late 2009) I noticed that the built in wireless would go out after a few minutes of operation and thereafter would reboot. Having tried all other suggestions here and other sites and opening up iMac to clean and check boards I found that turning off the Apple wifi resolved the repetitive rebooting. I installed a wifi adapter card and solved this iMac’s problem. Also found the Apple fan control very useful to keep temperatures down. Hope this helps.

I have the same problem with my mid-2011 27" iMac with the phantom restarts. This can be during startup, during the loading bar at the Apple logo, immediately after hitting the login button after typing my password and during use. I have noticed that it will restart, and then on the second boot it will last a little longer before restarting, and the third boot will last even longer….and sometimes 20+ restarts later it will stay on perfectly fine. If I put the iMac to sleep, the phantom restarts return (usually restarting straight away again). Same applies if I shutdown, let the HD go to sleep, allow the computer to load the screensaver etc. I have gone into power saving options and disallowed the iMac from going to sleep ever. This worked for a short while. and the iMac lasted longer without restart, but the phantom restarts returned after a week. So far I have tried: Replacing the RAM cards (twice with two different brands; Corsair & Kingston).Replacing the HD to an SSD.Replacing the fan near the CD Drive.Replaced the screen.Replaced the Wifi card.Replaced the Graphics card from another mid-2011 iMac entirely.Changed power cords.Changed power outlet.Changed the processor.Changed physical address and plugged the iMac in at a relatives.Changed country! (Yes, I was living in Thailand and now living back in the UK)Unplugged all additional peripherals such as USB etc.Installed various fan apps.Formatted the SSD and reinstalled MacOS.Formatted the HD and reinstalled MacOS.Downloaded MacOS to a USB stick and installed from that.Installed various versions of MacOS; Lion, High Sierra.Tried Disk Utility on startup to repair/recover. Apple have been no help what so ever as the iMac parts are no longer supported or maintained by Apple, and the default OS is no-longer supported. So they will not entertain me in the slightest. The only thing I have not yet tried is throwing the iMac off a cliff.

I read on this thread someone solved there’s by removing the Wifi card… so that’s my next attempt. Anyone else have any recommendations for me?

I have the same problem with a refurbed model I bought from BestBuy.ca with 4 years of Bestbuy warranty… The computer clicks and shuts off as if the cord got unplugged, then reboots. Sometimes happens during the boot process, other times during regular usage (though seems to be frequently triggered by GPU usage… but not consistent). Same results with Lion, Mavericks and Yosemite. Pretty sure it’s hardware. BestBuy Geeksquad replaced the logic board, and it still crashes. I, like you, have tried different cord, different outlet, with and without UPS, all with the same random results. Other 2008 iMac in same UPS is solid. I have to assume it’s the power supply. Going back to best buy tomorrow for them to send away again. Their no-lemon policy states that if they replace hardware 3 times, they’ll give me a whole new equivalent or new current model as a replacement. I’m looking forward to my brand new 27" iMac soon!!

Having same problems but it’s really weird since sometimes it starts like if it was new and sometimes it keeps restarting when I turn it on…. But then after several times it restarts by itself I can use it like normal… Second day with this problem and let’s until when my mood can take it… Closer Apple support is about 2 hours from here…..

My iMac 27 Intel Core i7 OS X Yosemite 10.10.3 Prosessor 3.4 with 2 HD discs keeps restarting everytime I turn it on. Sometimes it doesn’t get to the desktop before restarting, other times it restarts while I use it. When it restarts, it randomly shuts off and then boots again, sometimes over and over. But it doesn’t restart when using disk utility program. It just began after test & video card replacement at Apple store service (“Gatortech” in Gainsville, Fl.) The guy replaced video card and asked me to replace power inverter. I didn’t get it why and what for, until I got home and turn on iMac. Have tried everything: diagnostic hardware, reinstall OS to the initial, uncheck “Automatic restart…” , recovery both HDs, Mount & Unmount HD, even reformat drives making partitions - the problem exist. Any meaningful advice would be appreciated. Thanks

My 27" iMac. Model No. 814LL/A is crashing as well. My iMac keeps crashing and restarting, gets grey screen with pink stripes. Crash Report My issues started occurring after some recent Apple updates to El Capitan, about two weeks ago.

I’m having the same problem: Crash Report

I have the same problem. The repair guy said that it could be the problem of logic board, so I changed a new one, but not the graphic card. When I took it home, it went well for couple of days. But then, the same problem reappeared. And I changed different electric outlet and different cord, not working. I am wondering will change the graphic card work? It’s so frustrating. Update (02/27/2017) I think I got the problem solved. Originally, I got the same restarting problem. So I sent it for repair, and the shop told me that it was the logic board problem. I thought since I have to change the logic board, I should upgrade it at the same time. So I changed the logic board, CPU, and SSD hard drive, but not graphic card. When it came back, it worked fine for a couple of days, but the same problem came back. Then I sent it back for test, and the shop told me it might be the hard drive problem, so they changed it and sent it back. Unfortunately, the same problem came back in a couple of days. I was desperate, and ready to buy a new iMac. Luckily, I found this post: https://discussions.apple.com/thread/617… In the end, I asked the shop changing the PSU unit. It’s been a week, I never shut down the computer, and it never got one instant of that restarting problem.

I had the same problem with a Imac bought in 2017. Random restarts. After trying everything I knew, I took it to the Apple Store and they told me it was completely fine but when I brought it back, same issue. I tried to upgrade it to Mojave to see if that would clear whatever but it actually made it worse and would freeze and give me a black screen. After a downgrade I had an idea that it must have something to do with the graphics card possibly overheating( at times the fan would spin loudly during a freeze) Installed macs fan control and had it target the GPU diode and havent had a freeze since. I am an IT guy and this is a company computer but I really expected Apple’s people to know more than a random help desk dude.

My solution was cleaning the cooling vents to allow the system to breath — core temps didn’t approach the auto-cutoff. I suspect the interior boards and bits need a good dusting, too — that’ll be the next step if the exterior cooling doesn’t resolve things completely. Design flaw — too pretty for its own good so it can’t breath worth a dang — fans included.

I have the same computer as yours and the same issue for months. Last year I bought a new iMac to replace it (iMac Pro baseline) and since then I tried everything like you did to save the old one. I replaced the power supply as well. I detached original SSD and HD, once at time and both also, disconnected the motherboard, disconnected superdrive, tried to install a very old mac OS (Mavericks). I realize now it’s been 5 years ago for you, but I only have read now. The strange thing is the computer is behaving like when there is a power loss, but after that is restarting itself and at login (when it works) doesn’t say “Your computer shut down because a problem”. When is on and powered for at least 30 minutes, that trouble doesn’t happen anymore, and I tried the graphic test “Valley” for long time, with very intensive use of the GPU, so, GPU have to be ok. I’m giving up, though, don’t know what to try anymore.

https://support.apple.com/en-us/HT200553 This is Apple’s HOW-TO solve this problem. I think it’s better to print out and do the testing according to Apple’s step-by-step plan.

If your Mac spontaneously restarts or displays a message that it restarted or shut down because of a problem In rare cases, your Mac might spontaneously restart, become unresponsive, turn off, display a message that your computer restarted because of a problem, or display a message that you shut down your computer because of a problem. About unexpected restarts In rare cases, your Mac might encounter an unrecoverable issue affecting all open apps. When this happens, your Mac must be restarted. This is sometimes due to what is known as a “kernel panic” because an underlying part of the operating system (the “kernel”) has determined there is an issue that requires a restart. If your computer experiences a kernel panic, a message may appear for a few seconds explaining that the computer has been restarted: “Your computer restarted because of a problem. Press a key or wait a few seconds to continue starting up.” After a moment, the computer continues starting up. 

Preventing unexpected restarts In most cases, kernel panics are not caused by an issue with the Mac itself. They are usually caused by software that was installed, or a problem with connected hardware. To help avoid kernel panics, install all available software updates until Software Update reports, “Your software is up to date.” OS X updates help your Mac handle the kinds of issues that can cause kernel panics, such as malformed network packets, or third party software issues. For most kernel panics, updating your software is all you have to do. After your computer restarts Once your Mac restarts successfully, an alert message appears, “You shut down your computer because of a problem.”

Click Open to re-open any apps that were active before you restarted. If you believe the issue may have been caused by one of the apps that you were using, click Cancel instead. If you don’t click anything for 60 seconds, OS X automatically continues as if you had clicked Open. Note: If your computer is unable to recover from the issue, it may restart repeatedly, and then shut down. If this happens, or if you see the “computer restarted because of a problem” message frequently, see the Additional Information section of this article for guidance. Reporting the issue to Apple Once you log in, OS X lets you know that, “Your computer was restarted because of a problem.”

Click “Report…” if you want to see details related to the issue. You can also send these details to Apple. Sending these reports helps Apple to investigate the kinds of issues that cause panics to occur. Viewing the report may also provide additional clues as to what caused the issue. 

Note: If you find the term “machine check” in the “Problem Details and System Configuration” field of this report, it may indicate a hardware-related issue. See the Additional Information section of this article for guidance. Click OK to send the report to Apple, or close the window to dismiss the report. If the issue doesn’t happen again during the next few weeks, the issue is likely resolved. Software known to cause kernel panics OS X Mavericks helps you correct kernel panics related to software you may have installed. If the cause of the kernel panic is known, Mavericks offers to help you disable its related software: If “More Info…” appears, click it to see more details about the issue, including possible workarounds or resolutions.Selecting the option to “Ignore” does not alter the software that may be related to the issue.“Move to Trash” moves software that is likely related to the issue to the Trash, but the Trash is not automatically emptied. When you select this option, an additional sheet appears: Click “Restart” to disable the software that may be responsible for the issue.When prompted, enter an administrator name and password.Click “Move to Trash”.After restarting, the related software is in your Trash. Click the Trash icon in the Dock to see which software was removed.  Contact the developer of the software to see if an update or more information is available. Empty the Trash if you want to permanently remove the third party software. Additional Information Read the following information to learn more about diagnosing and troubleshooting a recurring kernel panic. Troubleshooting a recurring kernel panic Diagnosing a recurring kernel panic can be difficult. If you need help with this process, consider bringing your Mac to a Genius at an Apple Store or an Apple Authorized Service Provider for help.  If you plan to visit an Apple Retail store, you can make a reservation (available in some countries and regions only). Tip: To help diagnose recurring kernel panics, record the date and time it occurs, and any information that appears with the kernel panic message. Was the computer starting up, shutting down, or performing a particular task when the recurring kernel panic happened?Is the kernel panic intermittent, or does it happen every time you do a certain thing? For example, were you playing a particular game, or printing at the time?Does it occur only when a certain external device is connected, or a device is connected to a certain port? Isolate hardware or software as the cause of the issue To try to figure out if the issue is related to software or hardware, use the computer with a fresh installation of OS X on an external drive. Start the Mac from OS X Recovery.  If a kernel panic still occurs when started from Recovery, there is likely a hardware issue. See the “Hardware troubleshooting” section below for additional information. Open Disk Utility and use “Repair Disk” on your Mac’s internal hard drive (named Macintosh HD by default).

Important: If Disk Utility is unable to repair the internal drive, you should back up your important data immediately and if possible, reformat the drive. Consider bringing the Mac to a Genius at an Apple Store or an Apple Authorized Service Provider for further diagnosis. Be sure to ask that, if the drive needs reformatting or replacing, they contact you about escalating your case to a special data recovery service. If you plan to visit an Apple Retail store, you can make a reservation (available in some countries and regions only).   Connect an external drive with at least 10 GB of free space. Note: Make sure the external drive does not cause kernel panics, and is the only device on its USB, FireWire, or Thunderbolt port. Connecting the external drive and its cables to another Mac can help make sure the drive does not cause kernel panics.Install OS X on the external drive.Start up from the external drive.Use Software Update to install all updates until it reports “Your software is up to date.“Don’t install additional software on the external drive, but instead use the Apple applications to surf the web, view QuickTime movies, email, print, scan, and/or other activities. Continue using your Mac for the amount of time it would usually take for the issue to occur.If a panic occurs, select the “Hardware troubleshooting” section below to further diagnose the issue. If a panic does not occur, select the “Software troubleshooting” section below article to further diagnose the issue. Hardware troubleshooting Disconnect the external drive used in the above test to determine if the kernel panic is due to a hardware issue. Check peripheral devices first Go to the next section if you have no devices attached to your Mac. Turn off your Mac.Disconnect all peripheral devices. If you have a desktop Mac, make sure all you have connected is a display and Apple keyboard with Apple mouse or trackpad.Turn on your Mac.Use your Mac for the amount of time it would usually take for a kernel panic to occur.If a kernel panic does occur: Proceed the next section to check the internal RAM and third-party hardware. If a kernel panic does not occur: Power down the Mac and connect one peripheral device at a time and test until a kernel panic occurs. Note: A combination of peripherals may be the cause of a kernel panic. Disconnect one peripheral at a time to see if it causes a kernel panic by itself. If the kernel panic does not occur, continue to add peripherals until you find the other peripheral needed to cause the kernel panic. Check internal RAM and third-party hardware Turn off your Mac.Reseat the Apple RAM, and remove third-party RAM and third-party internal hardware.  If you do not have the Apple RAM that came with the system, reseat the third-party RAM.Turn on your Mac.Use your Mac for the amount of time it would usually take for a kernel panic to occur.If the kernel panic does not occur: The third-party RAM or internal third-party hardware may need to be replaced. If a kernel panic does occur:  Bring your Mac to an Apple Store, or an Apple Authorized Service Provider for service and support. Be sure to ask that, if the drive needs reformatting or replacing, they contact you about escalating your case to a special data recovery service. If you plan to visit an Apple Retail store, you can make a reservation (available in some countries and regions only). Software troubleshooting Disconnect the external drive used in the above test to determine the kernel panic is due to a software based issue. Start the Mac from OS X Recovery and reinstall OS X on your Mac.Start from the installation of OS X you just created.Use Software Update to install all updates until it reports “Your software is up to date.“Download and install any third-party software updates before reinstalling third-party software, especially drivers and kernel extensions.      Examples include: Virtualization softwareDrivers for add-on third party display cardsAnti-virus softwareNetworking software (especially software which enables third party network devices)Add-on file system support software; for example, software that lets your write to NTFS formatted media. If the issue continues, you will need to erase and install OS X as follows: Start the Mac from OS X Recovery.Complete a disk image backup via Disk Utility of the internal drive to an external drive with enough free space.Erase the internal drive using Disk Utility.Install OS X.Start from the internal drive.Use Software Update to install all updates until it reports “Your software is up to date.“Re-install your third-party apps and copy your user data from the disk image backup you created in step 2.  Note: Avoid copying data from the /Library and /System folders on your backup disk image. Advanced information about kernel panics and panic logs You can check kernel panic logs for more information. The kernel panic text is added to the log after you restart the computer, assuming that you did not reset PRAM (the kernel panic text is stored in PRAM until you restart). In Mac OS X v10.6 or later, the logs are located in in /Library/Logs/DiagnosticReports. Information that may aid developers in the investigation of a software issue may be in the log. The information may also provide clues as to what may have caused the kernel panic. Understanding and Debugging Kernel Panics – This technote addresses kernel panics: what they are, how to read panic logs and how to debug the code that caused the panic. Kernel Core Dumps – This technote explains how you can enable remote kernel core dumps used to collect data about the kernel panic. Note: If you are a software developer, booter settings and debug flags may cause different symptoms for kernel panics. Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Risks are inherent in the use of the Internet. Contact the vendor for additional information. Other company and product names may be trademarks of their respective owners. Published Date: January 14, 2018

Hi, I don’t know if I have the exact same problem but it looks like this. Every time I turn on my iMac, it shuts down during the boot process and restarts. Sometimes up to 10 times in a row. When the login screen is reached and I log in, the iMac also always shuts down within a few seconds to a few minutes. However, if I use this iMac in target display mode (cmd-F2) with another iMac, it no longer shuts down and I can use the iMac all day. If I log in to the iMac and then do nothing with it (no mouse movement and no keyboard), the iMac also stays on. Does anyone know what kind of defect is causing this? And why does the iMac never shut down (reboots randomly) when i use it in target display mode?? FYI the iMac on the left is a newer one with no issues. I usually (after a lot of spontaneous reboots) use the right iMac in target display mode as a second monitor for the left iMac. At 5.25min i perform a PRAM reset twice (without positive result). All reboots in this video recording occur spontaneously. I recorded my issue. You can see it on https://www.youtube.com/watch?v=eF6uiHwC…