iMac G5, 17-inch Troubleshooting -
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General Information
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.