Do both of these resets
- Shut down the computer.
- Unplug the computer's power cord and ALL peripherals.
- Wait 15 seconds.
- Attach the computers power cable.
- Wait another 5 seconds and press the power button to turn on the computer.
- The 5 second timing is important to a successful reset.
- Shut down the computer.
- Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on the computer.
- Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
- Hold the keys down until the computer restarts and you hear the startup sound for the second time.
- Release the keys.
I have exactly the same issue my friend. I have a Mid 2007 24" iMac and everything was well a few weeks ago. I installed Mountain Lion and after about 2 weeks of everything running smoothly, one day i turned it on and within 10 minutes the GPU diode temperature went to 120C and the GPU temperature at 100C.
I did everything you can imagine. I open the iMac myself (i had done that in the past too), took off the GPU, GPU diode and heatsink, i applied new thermal paste, but nothing really changed.
I ran the Apple Hardware Test and it gave the error 4SNS / 1 / 40000000: TGOH.
I read that it is the temperature sensor of the graphics card connected to the logic board (not the internal sensor).
I went back to Snow Leopard, did all the updates and now although the problem still exists, iStat shows 20C less than before when idle or doing simple tasks. GPU at 80C, diode at 92-96 and heatsink at 86C. If i open a youtube video, especially on HD, or if i try to play DIablo 3 or even Warcraft 3 the temperatures go crazy again, like with Mountain Lion.
Did you also install Mountain Lion before this happened? I don't think it's a random fact that we both have the same issue on the same model...
In a few days i'll go to a premium reseller, which is about to acquire Apple Authorized Service. If it is indeed just a problem of the sensor, i hope i won't have to replace the whole gpu. The iMac itself isn't worth that much now...
I'm having a similar issue with MacOSX 10.8 and GPU, but my temperature reading are lower (around 70 C). I do not have a comparison using 10.6, but what is strange to me is that CPU is a lot cooler, around 50 C: the two heatsinks are fed by the same fan and hence if the CPU is cooled, the GPU should be cooled as well.
I will try to vacuum-clean the iMac but I think it's mostly due to the GPU being stressed by the new Mac OS X revision. Best option would be to open the iMac and clean it thoroughly, but the risk is breaking something and/or dust between glass and LCD (I had the same dust problem with an old Nokia phone, but it's display was only 1,5" wide ... cannot imagine what can happen with a 20" display !)
I don't think you actually have an issue enryfox. 70C is not too much. Like you said, it's mostly due to the GPU being stressed by Mountain Lion.
As i said in my previous post, i went to an Apple Authorized Service Provider and they told me that everything seems ok. The temperatures are high by the imac is healthy. I could replace the GPU for 250€, but it's not worth it.
I got the imac back and i thought i should push it to its limits. So i opened Diablo 3 and played not minding the fans and the temperature. After a few minutes the mouse pointer went a bit crazy, couldn't control it much ingame. I minimized the game and iStat showed 126C for the GPU diode and 120 for the GPU. One or two minutes later the GPU temperature eventually reached 126C too and the iMac switched off by itself.
As i said i went back to Snow Leopard and when i don't push the iMac too much the temperatures are 90-95C for the diode and 75-80 for the GPU. I decided not too replace the GPU unless it dies and i will wait for the new iMacs. I still can't understand what caused the problem though, but cleaning the iMac from the inside and replacing the thermal paste on the GPU didn't solve it.
Are there any updates to this thread? I have a mid-2007 24" iMac with the Radeon HD 2600 Pro. I have taken my iMac apart 3-4 times and blew the dust off of everything and made sure all connections for sensors and cables were tight.
The problem persists:
My temp readings are (in celsius):
GPU Diode: 116
GPU Heat sink: 115
The CPU fan is running full speed all the time. I am running Mountain Lion.
I thought about buying a replacement video card from ebay, but was floored when I saw they were $250.
As i said in my previous post i went back to Snow Leopard and the idle temperatures dropped a bit when idle. I never played Diablo or Starcraft though because the iMac was shutting down after a few minutes.
I continued using the iMac for about a month and one day i left it in sleep mode. When i came back and woke it up and heard the fans, but the screen remained black. I turned it off and then it didn't turn back on (no startup chime). For some weird reason when i removed either of the RAMs it did turn on back the screen didn't show anything, so i suppose the video card is finally dead.
I really think Mountain Lion screwes up something with these particular iMacs. Although i went back to Snow Leopard the Problem persisted.
A few days ago i bought a new Macbook Pro 15", but i'm still thinking of repairing the iMac. The Authorized Service Provider i went to told me the replacement of the video card will cost me 250€. I can't find it cheaper elsewhere, in order to do it myself, so i'll probably let them do it.
It's a pity, because the iMac was still going strong. Be careful cause yours may be end up in the same condition as mine!
Your experiences are almost exactly the same as mine. Mine just has not progressed to the point of being completely dead. I dont think it is far from it though.
I was thinking of trying a couple cheap fixes before giving up on it completely.
I thought of maybe replacing the sensor, re-applying thermal grease, and then posisbly replacing the heat sink.
If it gets to where I have to replace the GPU or logic board, each of which are $250 parts, I would be better off selling it for parts and buying a new computer. Like you, I thought it still ran good, so am not quite ready to give up on it.
I feel you completely my friend. If you read my first post on this thread, i said that i applied new thermal grease too. I didn't change anything though. Only going back to Snow Leopard helped a bit (temperatures dropped by 15-20C when idle), but nevertheless they were still high and the graphics card ended up dead.
Did you run Apple Hardware Test too? It gave me this error as i wrote previously: 4SNS / 1 / 40000000: TGOH.
It is supposed to be a sensor problem, but if it really was i don't think the GPU would give up.
If you can find a new sensor or heat sink try it. You never know! If i decide to give it for service and they tell me it's not only the GPU, but the logic board or both i think i'll sell it for parts too. If it's only the GPU i'll probably end up paying the 250€.
The problem is they are old models and Apple won't recognize anything even if all the Mid 2007 iMacs have the same issue!
I did get an error from the Apple Hardware test, but I cant remember what it was. I will see if I can dig up the OSX disc tonight so I can run it again. Since I upgraded my OS, it doesnt work with the normal keyboard shortcut.
I also just bought a used heatsink off ebay for $10. I am not so sure the heatsink will do much, but for $10, it is worth a try.