Da der Mac wohl ein Problem mit der Grafikkarte.
iMac Intel 24" EMC 2134 and 2211 Diagnostic LED's [ifixit.com]
» LED 3 - Indicates that the computer and the video card are communicating.
» This LED will be ON when the computer is communicating properly with the
» video card. If LEDs 1 and 2 are ON and you heard the startup sound, but
» LED 3 is OFF, then the video card might be installed incorrectly or need
» replacement.
» LED 3 - Indicates that the computer and the video card are communicating.
» This LED will be ON when the computer is communicating properly with the
» video card. If LEDs 1 and 2 are ON and you heard the startup sound, but
» LED 3 is OFF, then the video card might be installed incorrectly or need
» replacement.