iMac shuts down by itself, won't restart

It looks to me like it is due to overheating maybe? Download Temperature Monitor and if you see that the temp gets high, perhaps you should download smcFanControl to rev those fans up a bit more than the default settings.

Does this happen when you use a particular application or is it happening no matter what you are running? Do you clean out regularly the air vent in the back where fresh air enters your iMac to keep it cool? For example, I have two cats and cat hair sticks to the opening, I have to get it off so the air vent won't be blocked. It could be an accumulation of dust inside the case as well.

I had a similiar problem, twice in the last year. Mine, however, would restart my itself most of the time. Of course I tried several suggested fixes that essentially just wasted my time. Both times new logic and power supplies fixed the issue.
Just thought I would let you know so you don't waste too much time.

I had this happen on a Core Duo 17". After months of troubleshooting and replacing a power supply (not easy to find online), it was a loose connection to the logic board from the power supply. It's very difficult to get to/diagnose as you have to remove the entire logic board to see behind if the plug is loose. That was all that was wrong, simple as that. I tried unplugging temp sensors, SMC Fan Control, everything for two months...it was just a loose power connection in the end. If you can find the time to take it apart (carefully) and double check, you may save yourself some $.

If you have the 2.40 version iMac freezing is a "known Issue" according to APpleCare. I went through 4 of this model until I got pucky they upgraded the model to 2.60 and changed the flawed ATI video card.

Same problem here with an early-2006 Intel iMac...

It started with problems turning it on but leaving the power cord disconnected for 10/20 seconds solved it (So this is not an overheating problem since it occurred before using it) then, all of a sudden, the iMac shutted down while waking up from sleep...

Since then it shuts itself down after 20/30 minutes... I have the BOINC client running, so the CPU usage is almost always 100% (Today i'm going to clean it and try to close BOINC to keep CPU usage at minimum... Last time i tried i gave a look at the temperatures (70 degrees celsius for the GPU, 40 for the HDD, something between 40 and 55 degrees celsius for the other components) and fan speeds (Don't remember but something like 1200-1500 rpm) but havent's found anything really unusual or awfully high...

I'm not sure if this is an overheating or a PSU problem, but the only "Authorized Support Center" here really suck... I'll let you know if my tests will give me some answers...


Cheers!
Luca Nonato

Is there a record of this"known issue" somewhere I can print out and take in to the Apple Store with my Mom's iMac? Same problems as yours. - 2.4 version.

The temperature sensor is connected to one of the heat tubes which transfers the heat away from the processor to where the heat is dissipated through the cooling fins. However the intel processor has an internal thermal protection which will shut itself down in the event of over heating, and the system will never know exactly what the actual temperature of the processor is. In other words, it is possible that if you have a poor interface between the processor die, and the heat sync, you will measure a lower temperature than the processor actually is experiencing, and still have a forced shut down issue.

I also am running boinc to see if this is a heat issue with the fans running full speed. I performed the same test yesterday without a SMC hack running. Today, I have SmcFanControl running with the fans running at full speed.

I'm not sure of your exact situation, but in my case and with other cases I have experienced, the shut down time interval decreases with each shut down, which is very indicative of a thermal issue of some sort. In my case, if after today there is no shut down, I would say that the SMC is not correctly managing the temperature by controlling the fan speed. If it does shut down, I suspect an interface issue between the logic board and the processor. I am a certified apple tech, so for me, what I am about to suggest will not void my warranty on the computer I'm servicing. If your computer is not under warranty, by all means go ahead. It's not any good if you can't even turn it on. Anyway, if the computer still shuts down, I will remove the processor heat sync and make sure the thermal paste hasn't "oozed" so to speak off the processor die necessitating reseating the heat sync with a fresh application of thermal paste. I have been reading of people getting their logic boards replaced to fix this issue, which also requires a reseating of the heat sync, so I think people may be replacing good logic boards with good logic boards. The only defect appears (from my reading) to be the SMC mismanaging the fan speed. I say this because my initial test I performed with Boinc running, the measured temperature rose and rose, but never got to a really high temperature. But the fans never sped up either, and eventually after about 7 hours of running boinc, it started it's spiraling shut down trend until it wouldn't even get to POST. I tried resetting the PRAM, which it shut down after the fist POST chime, then it never got to POST.

I have also had computers where the SMC reset fixed the issue, and I have those computers in service again without any issues.

My point:
Gather all the information you can. Posting a couple of things you tried, without trying everything doesn't help the trouble shooting process a whole lot.

Hope this helps:)

FYI:)

The computer did shut down sometime overnight. Now I will disassemble the computer and reseat the heat sync. About 2:00pm yesterday I turned control of the fans back over to the SMC. I also updated the OS, no change.

As one more final trouble shooting effort, I left the computer running all weekend with boinc running. No shut down.

After opening the computer to reseat the processor heat sync, I discovered that apple does not allow for the separation of the heat sync from the processor. It will void the warranty. So I wound up ordering and replacing the entire logic board. I understand now why others have done this, but I don't understand the power supply. If you follow apple's recommended course of action for repair technicians, it says replace the DC cable first, not the power supply.

I've just started Boinc on it again with the SMC solely controlling the fans. Already I can see a difference. The fan speed immediately sped up to 2300 rpm rather than sit at idle. I will continue to test this way for another 20 hours or so.

I have another of the same model that was purchased at exactly the same time, I've been running boinc on it since I made my first post here, and it has not experienced a single shut down while the SMC is controlling the fans.