Mac Mini fan won't turn off (only while powered on)

Have you turned it over and vacuumed out the air vents? It sounds like it may be running hot.

Luck-

-DP

When you start your mini from cold (after at least an hour powered down) does the fan spin up to full speed immediately, or does it start out normally and then speed up after a few minutes or so? Did you have the system open for any reason just before this behavior started?

If you had the system open, it might be something as simple as the fan control cable that is not connected. When disconnected, the fan will run at full speed all the time.

It the fan runs at full speed almost immediately from cold, then it would suggest one of two things: either the system is suffering from something like a loose heatsink, or there is an errant process running that is putting the system under heavy load. You can check that by opening Activity Monitor (in Applications>Utilities) at looking to see if the CPU load is consistently high. If it is, look for a process that is causing it.

If the fan runs quietly at first, and then speeds up, after a few minutes, then it does seem likely to be a heating issue. Aside from making sure the vents around the base and on the back are clear by using a good vacuum cleaner, it's possible there is enough debris inside to obstruct the airflow. If you've opened up the system previously for any reason, now might be a good time to do so again, for a quick inspection and clean out.

I would also perform an SMC reset if you have not already done so:
To reset the SMC on an Intel mini:
-From the Apple menu, choose Shut Down (or if the computer is not responding, hold the power button until it turns off).
-Unplug all cables from the computer, including the power cord and any display cables.
-Wait at least 15 seconds.
-Plug the power cord back in, making sure the power button is not being pressed at the time.
-Then reconnect your keyboard and mouse to the computer.
-Press the power button on the back to start up your computer.

And to ensure you are not suffering from a range of broad 'tuning' type issues, download a copy of YASU ( http://jimmitchelldesign.com/projects/yasu/) and run all tasks. This clears cache files, resets logs files, updates prebinding, performs a permission repair and a few other tasks. After the reboot (which will be slower than usual) your system should run a little better.

... i don't know if the Apple Store would look at it for free if i brought it in....

You can make an appointment at the 'genius Bar' of your local Apple store - they will look at your system for free, and help with a diagnosis. What they generally wont do without charging for it is anything that involves opening the system up. However, it's worth paying them a visit for a bit of hands on work!

has it ever been opened?

as you say the front light, and the bluetooth dont work either?

if yes, pop it open again and undo the screws and flip the hdd/disc drive over to expose the cpu heatsink and ram. at the front there are two little black two-wire connectors, facing you the left one is the fan sensor and the right is the light. make sure these are properly connected.

even if no, maybe try that anyway

The fact thew fan comes on immediately from cold is a relatively good sign, because it means the system is not suffering from overheating. And your description of what you see in Activity Monitor may mean little to you, but perfectly describes a system which is running as expected and not under the kind of load that would cause the fan to spin up to full speed.

Those two factors together would then lead to the suspicion that either the SMC is misbehaving (it's the system management chip that controls power and function of the system overall, including the fan) or that the fan control cable has disconnected.

The SMC reset will not risk your files or any other data - it resets the controller but nothing else - so there's no danger of having to do anything like rebuilding your playlists when your mini was put into service.

If the SMC reset doesn't resolve the problem, I think it may be time for the top to come off the mini for the fan control cable to be checked.

I had the same problem start up suddenly last week-end.
I rebooted and it got quite for a while, then went to full speed again.

It's not dusty at all, but I blew it out just to make sure.

But, I applied patches today and noted that the OS is 10.5.6 now (it was 10.5.4) and the fan hasn't come back on like it did before.

I also noted that my laser printer is behaving better - no more printing delays.



QUESTION for AndyO: What is SMC and how do your reset it ?

SMC reset procedure described here:

http://support.apple.com/kb/HT1543?viewlocale=en_US