Check the RAM, you may have a loose or partially seated module. Remove and reinstall the modules pushing real hard on them with both thumbs to insure that they are fully seated in the slots.
Then if it is still acting up, run the Extended Apple Hardware Test from the System Disc that came with that iMac. > Intel-based Macs: Using Apple Hardware Test
Thanks for the reply.
I checked the RAM, removed both modules and reinstalled them and that didn't work.
I just ran the basic Hardware Test and that didn't come back with anything either. You think the Extended would provide something different?
I have a wireless mouse and it wasn't working (not sure if that's normal or not, assuming it is) so I just tabbed over and hit enter to run the basic.
Run an extended test, this might take a long time. AHT (Apple Hardware Test) is what AASP will do when you give the product for service. Extended test will check each component, you will find some errors if issue with h/w. If not logic board is faulty. Also check for cooling solutions, iMac has major issue with fans. If iMac is over heating, frequent shutdown is common.
So, the computer didn't act up for a while but here we are again getting the same problem, only more frequently.
I tried to run the extended test a while back and of course I can't use the wireless mouse to select extended test.
As it hasn't been a huge problem I just left it be.......that and the original disc was in storage somewhere.
Anyway, today I had enough. I found the disc the other day because I just moved and ran the extended test.
Came back with:
Looking around and people are saying the fan is dead. Sure enough, checked fan speed on the HD with iStatPro and it is at 0........guessing that means dead.
Wondering if it is worth it to replace the HD itself along with the fan. Any thoughts? And what's the easiest way to transfer data to the new HD?
I have an iMac circa 2009. Have been experiencing the same spontaneous shutdowns.
After much research I found an article by an Apple engineer (sorry, failed to bookmark) which explained the cooling system of the iMac. These computers are not cooled directly by internal fans, rather by ambient ventilation and heat displacement. Internal fans are most efficient when speeds are at factory setting. The use of any kind of external fan only interferes with the system's cooling machine. Lastly, screen brightness should never be set at or near full brightness as heat from the screen only adds to the general temperature of the machine.
Implementing these 3 suggestions solved my spontaneous shutdowns. Using Fan Control (not iStat) I saw my CPU, HD, GPU temps drop by almost 10 degrees C. Lion's a tad more spunky. Most importantly, my computer isn't shutting down anymore as if someone had pulled the power cord.
Over the past several months that I have tried to resolve this problem I have done everything from completely disassembling the machine, cleaning fans, checking heat sensors, switching RAM, power cords, uninstalling suspicious apps, all to no avail. In other words, this has been a long, nerve-wracking process for me. Not understanding at all how an iMac really manages heat displacement and prompted by some misleading suggestions to ramp up fan speeds, I was compounding the heat issue problem.
This resolution may not work in your case, but it did for me. Please post comments if this helps your shutdown problem.