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:
Disconnect all external devices from your iMac, except the keyboard and mouse.
If you have an Ethernet cable connected, disconnect it.
Restart your iMac while holding down the D key.
When the Apple Hardware Test chooser screen appears, select the language you
want to use.
Press the Return key or click the right arrow button.
When the Apple Hardware Test main screen appears (after about 45 seconds), follow
the onscreen instructions.
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.
Problem, Meet Solution
If the Apple Hardware Test window doesn’t appear after restarting, hold
down the Command (x), Option, and D keys and restart the computer to access the
Apple Hardware Test software from the Internet. Your computer must be connected to
a network that has access to the Internet.