Apple iMac G5, 20-inch - Manual (Page 248)

24 - iMac G5, 20-inch Troubleshooting Diagnostic LEDs
7. Locate the large white arrow in the middle of the board. Above this arrow, you'll see
four LEDs (explained below).
• LED 1 indicates that the trickle voltage from the power supply has been detected
by the main logic board. This LED will remain ON whenever the iMac G5 is
connected to a working AC power source. The LED will remain on even when the
computer has been shut down or put to sleep. The LED will turn off only if the AC
power source is disconnected or the power supply is faulty.
• LED 2 indicates that the main logic board has detected proper power from the
power supply when the computer is turned on. This LED will be ON when the
computer is turned on and the power supply is working correctly.
• LED 3 indicates that the computer and the LCD display are communicating. This
LED will be ON when the computer is turned on and video signal is being
generated.
• LED 4 lights only if the computer detects an over-temperature condition. This LED
will be OFF when the computer is turned on and running at the correct
temperature.