My Mac Pro 2009 fans start but doesn't boot

@mstreem

REset your SMC first

Resetting the System Management Controller (SMC)

The System Management Controller (SMC) controls all power functions for the computer. If the computer is experiencing any power issue, resetting the SMC may resolve it. The SMC controls several functions, including:

• Telling the computer when to turn on, turn off, sleep, wake, idle, and so forth

• Handling system resets from various commands

• Controlling the fans

It is also recommended that the SMC be reset after a backplane board or processor board is replaced as part of a repair.

Note that resetting the SMC does not reset the PRAM. Resetting the SMC will not resolve issues in which the computer is unresponsive—in these situations, restarting the computer will generally suffice. If the computer isn’t responding, perform these steps one at a time, in the following order, until the issue has been resolved:

1. Force Quit (Option-Command-Escape)

2. Restart (Control-Command-Power)

3. Force Shut Down (press the power button for 10 seconds)

Resetting the SMC can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so forth. If the computer still exhibits these types of issues after you’ve restarted the computer, try resetting the SMC by removing AC power:

1. From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off).

2. Unplug the AC power cord.

3. Wait at least 15 seconds.

4. Plug the power cord back in, making sure the power button is not being pressed at the

time. Note: Ensure the power button is not pressed when connecting the AC power cord or all fans will run fast.

5. Press the power button to start up the computer.