The Mac Pro logic board includes a set of LEDs to help service providers troubleshoot the
computer. The LEDs are located toward the rear of the logic board, under the memory cage, next to PCI card slot #1.
You can view these LEDs by removing the computer’s side access panel and looking through the memory cage to the logic board below. ***LEDs 2, 3, 4, and 5 are normally off and will automatically illuminate if an error occurs.*** ''To read LEDs 1, 6, 7, and 8, you must ***press the DIAG_LED button***, which is adjacent to the LEDs'' (white button to the right). To press the DIAG_LED button, use a nylon probe tool.
***Power Supply Verification***
To power on, the computer’s logic board requires “trickle” power. If the system fails to power on, first reset the SMC. If the computer still doesn’t power on, follow the procedure outlined below to determine whether the issue is related to the power supply.
Verify trickle power
***Diagnostic LED 1*** indicates the presence of trickle power required by the logic board to begin the startup process. ''LED 1 should be yellow when the DIAG_ LED button is pressed, indicating that trickle voltage is present.''
***Verify Power Supply Is Providing Power***
Diagnostic LED 7 indicates that the main power is OK and within regulation. Plug in AC power cord, and press the power-on button on the front panel. ''LED 7 should be green when the DIAG_ LED button is pressed, indicating that the main power is OK and within regulation.''
There is a "backup battery" on that board- it may need replacing.
''If this answer is acceptable ***please remember to return and mark it***.''