The SMU (System Management Unit) is a microcontroller chip on the logic board that controls all power functions for your computer. If your computer is experiencing any power issue, resetting the SMU may resolve it. The SMU controls several functions, including:

- Telling the computer when to turn on, turn off, sleep, wake, idle, and so on.
- Handling system resets from various commands.
- Controlling the fans.

Note that resetting the SMU does not reset the PRAM. Resetting the SMU will not resolve issues in which your computer is unresponsive\ufffdin these situations, restarting your computer will generally suffice. If your computer isn't responding, perform these steps one at a time, in this 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 SMU can resolve some computer issues such as not starting up, not displaying video, sleep issues, fan noise issues, and so on. If your computer still exhibits these types of issues even after you've restarted the computer, try resetting the SMU. To reset the SMU on one of these iMacs:

1. Turn off the computer by choosing Shut Down from the Apple menu, or by holding the power button until the computer turns off.
2. Unplug all cables from the computer, including the power cord.
3. Wait 10 seconds.
4. Plug in the power cord while simultaneously pressing and holding the power button on the back of the computer.
5. Let go of the power button.
6. Press the power button once more to start up your iMac.

Needless to say, this odd dance must needs await my return from San Francisco at the close of the workday. One hopes, of course, that the issue is so easily resolvable, but it is still off-putting to have to confront it at all.

cordially,

—the operative word being "roaring" at first. The SMU reset worked as advertised, except that when the machine revived, its formerly whisper-quiet cooling mechanism sounded more like an Airbus A300 as heard from inside the passenger compartment. Another call to Apple and between a brisk round of permissions verification and repair and an attempted reboot into "open firmware" (which didn't appear to take) we were yielded up a to a proper desktop with the blessed Silence of the Fans, and that's where we are right this moment. I must say, though, that I feel a little like the guy who adopted the cute puppy only to find that it will not be housebroken...

cordially,