So in addition to being one sweet machine that can do all sorts of crazy things, the iMac can also provide key troubleshooting clues via diagnostic LEDs on the main logic board.
A WORD TO THE WISE: These Apple-provided steps do involve removing the back of the iMac, which involves a Phillips screwdriver (#1 size), towel or soft cloth for cushioning the iMac on a flat surface, and not touching the computer’s insides after running around in socks or something like that. That way lies badness.
But once the iMac’s back cover is off, one should see a large white arrow in the computer’s middle section and below it, four LEDs. But what do they mean?
And there you have it. A little computer troubleshooting for a little elbow grease (minus any actual grease. I hear that’s bad for computers).
LED 1 indicates that trickle voltage from the power supply has been detected. This LED will be ON when the computer is turned off and your power supply is working correctly.
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.
