If the computer boots after you press the SYS_RST switch, try shutting the computer off and restarting by pressing the front power button. To press the switch, use the nylon probe tool (Apple part number 922-5065). With the computer powered up, press the SYS_RST switch located in the upper right-hand corner of the logic board. This switch can be used to determine if a computer that won't consistently boot from a cold start has power supply issues. Mac Pro (Early 2008) has a system reset switch on the logic board that may be used to restart the system while it is powered up. You may need to remove a PCI Express card to have access to the battery. Remove the battery for at least 20 seconds. If the computer is experiencing an issue booting, resetting the RTC may resolve it. The Real Time Clock (RTC) is a chip on the logic board that controls the date and time functions of the computer. Note: Unlike the logic board in earlier Mac Pro models, the logic board in the Mac Pro (Early 2008) computer does not include an SMC reset switch. Press the power button to start up the computer. Plug the power cord back in, making sure the power button is not being pressed at the time. From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off). If the computer still exhibits these types of issues after you've restarted the computer, try resetting the SMC by removing AC power: Resetting the SMC can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so forth. Force Shut Down (press the power button for 10 seconds). If the computer isn't responding, perform these steps one at a time, in the following order, until the issue has been resolved: 
Resetting the SMC will not resolve issues in which the computer is unresponsive-in these situations, restarting the computer will generally suffice. Note that resetting the SMC does not reset the PRAM.
It is also recommended that the SMC be reset after a logic board, ambient board, or ambient board cable is replaced as part of a repair. Handling system resets from various commands.Telling the computer when to turn on, turn off, sleep, wake, idle, and so forth.The SMC controls several functions, including: If the computer is experiencing any power issue, resetting the SMC may resolve it. The System Management Controller (SMC) is a chip on the logic board that controls all power functions for the computer.
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