Hello MacRumors! I am posting here for the first time because I am handling a rather tricky case here. My girlfriend's iMac (21.5-inch, Late 2012, running Maverick) has been displaying some issues as of late. Particularly, it'd refuse to turn on from time to time. Also, if she puts it to sleep at the end of the day, the iMac will be switched off by morning and will refuse to turn on. Pressing the power button, the keyboard and the mouse does nothing to power the iMac up. The strange thing, however, is that if she lets the problem sit for a day, for example, the iMac will come back to life for a few hours to a few days before the same thing happens again. I have tried a couple of things, and I will list them down: 1. SMC and PRAM resets 2. Apple Hardware Test (No problem detected whatsoever) 3. All the checks in Disk Utility and Onyx. 4. Removed all third-party hardware to isolate the problem. It's still happening. Now, at this point, I figured it is probably a hardware problem not previously detected by the Apple Hardware Test. I figured it is probably the Power Supply Unit being faulty, or perhaps the cable. However, I noticed something strange just yesterday. The iMac is able to run the entire day if I force it to run processes that require the computer to be turned on. For example, backing up via Time Machine, as well as downloading movies. These processes seem to keep the iMac turned on without random shutdowns. I have a feeling that, because of this, it is not a hardware issue that's related to power supply at all. The one thing that I have yet to try yet is reinstalling Maverick via a bootable drive or Maverick's in-built reinstallation feature. Do you guys think that a clean reinstall (with a Time Machine backup of course) may potentially solve the problem? Thank you so much!