The iMac G5 has built-in diagnostic LEDs on the main logic board that can help you to troubleshoot your computer.
To access these LEDs, remove the back cover of the computer:
- Turn your computer off (choose Shut Down from the Apple menu). If you can't shut it down that way, you can turn the computer off by pressing the power button for five seconds.
- After the computer is off, disconnect all cables and the power cord from your computer.
- Place a soft, clean towel or cloth on the desk or surface. Hold the sides of the computer and slowly lay it down so the screen is flat against the surface and the bottom is toward you.
- Raise the foot and use a Phillips #1 screwdriver to loosen the three screws at the bottom of the computer by turning them completely counter-clockwise. These are "captive screws," which means they're not supposed to come all the way out.
- As you hold the metal foot, tilt the back cover up and lift it away from the computer. The back cover with the foot attached should come off easily. If it doesn't, make sure the screws are turned completely counter-clockwise, but not too far.
- To protect the computer from electrostatic discharge, ground yourself by touching a metal surface inside the computer. Warning: Always discharge static electricity before you touch any parts or install any components inside the computer. Don't walk around the room in socks or bare feet, for example, until after you've finished these steps.
- Reconnect the power cord to the back of the computer.
- Locate the large white arrow in the middle of the computer.
Above this arrow, you'll see four LEDs:
- 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.
These diagnostic LEDs can be used to help you troubleshoot your computer.