21" iMac keeps shutting off during use..

[HE15MAN]

This is the 5th time it has done this. 3 times in the last week. Now, there is a LOUD fan noise running constantly.

What could be wrong? It was purchased approx 2 years ago from Best Buy.

Thanks

[Vylen]

This is the 5th time it has done this. 3 times in the last week. Now, there is a LOUD fan noise running constantly.

What could be wrong? It was purchased approx 2 years ago from Best Buy.

Thanks

Overheating.

Loud fan noise is your iMac working as hard as it can to cool itself down.

But it's puny fans aren't enough and so the system shuts down to protect itself once it reaches critical.

[Sun Baked]

There still should be hardware diagnostics on the System Restore disk you got with the machine.

Restart and hold down the d key to run the diagnostics off the System Restore DVD.

Likely a fan or a sensor is bad.

[666sheep]

Try SMC reset first: http://support.apple.com/kb/HT3964 - it's recommended for problems like yours.
If it won't help, do Hardware Test like mentioned above.
If you'll get any error code during Hardware Test, post it here.
You could also check your temps, with iStat or Temperature Monitor.

[kmgordon]

My 20" has been shutting down on its own for a few months. There is no rhyme or reason. Might be on the net or on any application. Just shuts down as if the plug was pulled. Immediately starts back up when hitting he power button. I took it in to apple, they had it 6 days and said they have no idea what it is as it didn't happen while they had it. Once home it did it again, about once every few days. Today it was 4 times. I've done everything I can find with suggested fixes and still occurs. Not sure what else to try. I've owned macs since 1984 and never had such an issue.

[purpleclad]

My brand new 27" i5 is doing this exact same thing. Happened to me 3 times within the first 45 minutes out of the box. Apple Care had me do a couple of resets to the system, but it still is doing it. They are shipping me a new one out on Monday.

[Thermonuclear]

Use the Console application to see if the system log has anything to say.

[ranosgol]

I had the same issues with iMac G5 shutting off. The power supplies are usually the issue they fail and need to be replaced.

When I replaced mine the issue never returned and I had it on two different models.

[ezramoore]

I had the same issues with iMac G5 shutting off. The power supplies are usually the issue they fail and need to be replaced.

When I replaced mine the issue never returned and I had it on two different models.

While I wouldn't rule it out in this case, this issue was pretty isolated to the G5.