Apple iMac (21.5-inch and 27-inch, Mid 2011) - Manual (Page 55)

55 Chapter 4 Problem, Meet Solution
7 To reinstall Mac OS X and your applications, follow the instructions in the
Mac OS X Installer.
After reinstalling Mac OS X, you can re-download the Apple applications that came
with your iMac and other apps you purchased from the Mac App Store. You can restore
your other data and applications from a Time Machine or other backup.
Checking for a Hardware Problem
If you suspect a hardware problem, use the Apple Hardware Test application to help
determine if there’s a problem with one of the iMac components.
To use Apple Hardware Test:
1 Disconnect all external devices from your iMac, except the keyboard and mouse.
If you have an Ethernet cable connected, disconnect it.
2 Restart your iMac while holding down the D key.
3 When the Apple Hardware Test chooser screen appears, select the language you
want to use.
4 Press the Return key or click the right arrow button.
5 When the Apple Hardware Test main screen appears (after about 45 seconds), follow
the onscreen instructions.
6 If Apple Hardware Test detects a problem, it displays an error code. Make a note of the
error code before pursuing support options. If Apple Hardware Test doesn’t detect a
hardware failure, the problem may be software-related.