MacPro a1186 won't power up. when pressing the serv_diag botton, i only get one yellow light (indicating trickle power). the final diagnosis from the service source manual (sections are included below) is less clear to me. if i only get trickle power to the logic board, and the power connections check out, is it the power supply or logic board failure? could it be something else? Power Supply Verification To power on, the computers logic board requires trickle power. If the system fails to power on, first reset the SMC as described in this chapter. If the computer still doesnt power on, follow the procedure outlined below to determine whether the issue is related to the power supply. Verify trickle power Diagnostic LED 2 indicates the presence of trickle power required by the logic board to begin the startup process. LED 2 should be yellow when the DIAG_ LED button is pressed, indicating that trickle voltage is present. Verify Power Supply Is Providing Power Diagnostic LED 8 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 8 should be green when the DIAG_ LED button is pressed, indicating that the main power is OK and within regulation. If diagnostic LED 5 and LED 6 are both solidly illuminated at power up, improperly seated power supply cables could be one cause of this behavior. Check all power supply cable connections to ensure they are properly seated. Startup Failures Power-on LED does not illuminate when power button is pressed, fans do not spin, and there is no boot tone or video Verify power outlet is good. 1. Check that diagnostic LED 2 is on when the power cord is connected.2. Replace power cord. 3. Check that diagnostic LED 2 is on when the power cord is connected.4. Reset the logic board. Refer to 5. Resetting the Logic Board in this chapter. Verify power supply cables are fully connected. 6. Mac Pro (Early 2008) Troubleshooting Symptom Charts 165 Verify that processors are properly seated. Check diagnostic LED 3 and/or LED 4. 7. Verify that processors mounting clamps are properly tightened. 8. Test whether the front panel board or power button is at fault. Remove the installed front 9. panel board and test with a known-good front panel board. Replace front panel board. 10. Replace power button. 11. Replace power supply. 12. Replace logic board.