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Model A1200 with 2.16 or 2.33 GHz Core 2 Duo processor

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Won't Boot Past Apple Logo, White Screen, HELP!

I have a late 2006 24" iMac with the NVIDIA GeForce 7600GT, 256MB. I recently have a problem where it doesn't boot past the white screen with the apple and activity circle. When they disappear, I am left with a white screen.

I booted into diagnostics and got the error code: 4SNS/1/40000000: TG0H. Not sure if this is the problem or not but I thinking the video card is the problem or a sensor on the video card.

Under diagnostics, when I pull up the Hardware Profile, it knows the resolution and memory size of the card but lists "unknown video" under video type. I did all the reset stuff and tried booting from the installer disc, etc... No luck.

I have replaced the main logic board and I still have the same problem.

Now the problem is, I can obtain a new video card, no one seems to have them in stock and say they are on back order from Apple with no date as to when they can get them. What else can I use to get this thing running or does anyone have any other ideas as to what the problem could be? Thanks in advance for your help.

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According to my source that code indicates a problem with the left heat pipe. Did you reapply thermal paste when you replaced the logic board? The error code would seem to indicate a heat problem so you might recheck all sensor connections. I'd wait for mayer to advise you on the video card.

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It appears the horse is already out of the barn on this one. You've replaced the logic board without even knowing what the problem is. Let's start over. When you boot the machine from your system installation disk are you reading the results from your display or an external display?

This machine did have the 7600GT as an upgrade at the time of purchase.

Although Apple makes no mention of it in technical documents, a number of third-parties have reported that the video in the White iMac "Core 2 Duo" 2.16 24-Inch system uses a MXM module, so it theoretically could be upgraded, although to date, MXM modules for purchase by individuals are few and far between.

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I replaced the logic board because at the time I wasn't sure what the problem was so I ordered both the logic and video card. Well the logic board came and the video card was on backorder so I figured I'd rule it out by swapping it. It did not make a change in the problem so I sent it back. This was done well before I knew about how to boot into diagnostics.

I still have the same problem as mentioned above and think it could be the video card, it is the only thing left.

I was reading the results from the built in 24" display. I can run the diagnostics but trying to boot from the install disk as to try to reinstall the OS I can not do, nor safemode, I get same problem as mentioned above.


Are you telling me the screen is fine in diagnostic mode? Is there any way you can hook up an external monitor? I know you think you have a hardware problem, but I've heard nothing that convinces me of that.


I tried hooking up to my 46" TV via HDMI converter from back of iMac but I had no signal out according to my TV. I wasn't sure if there was something I had to do to get the video to be external other than to just hook it up. Is there a key stroke I need to do to get it to switch to the external monitor while booting?

Normally, I can't boot into anything other than Command S and Diagnostics, everything else just gives me a white screen after the apple disappears (around 10-20 seconds). I can still hear the hard drive running in the background as it is probably continuing to boot but only have a white screen.

My thought is that the video card works ok under low res but as soon as it requires high res it just doesn't put out and I get the white screen. Does that sound logical?


I wouldn't count the TV as a reliable test unless you've done it before you started having problems. I think there's a very good chance that your problems are software related.


As a side note, I really appreciate you taking the time to help me on this one. I am so frustrated at this point. I was a PC guy since the Mid 80's, building systems on the side for friends, co-workers, and family. My brother-in-law talked me into Macs in 2006, hence my late 2006 iMac and I loved them ever since. I try to stay away from PC's as much as possible but I still work on them for people on the side for old times sakes. Anyway, I am still learning about Macs and how they work and all the ins and outs so that is why I may sound somewhat less educated with probably common Mac commands and abilities. I actually went out and bought a used Mac Mini to use in the mean time (Mid 2010) in hopes I can get my 24" one fixed up. Well at least until the end of the year when I can hopefully afford a new 27", wife willing... ha ha...


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I'm having the exact same issue with the same iMac.

Won't boot past the spinner. Tried everything that Steve did with no luck..

Did Steve ever get this resolved?

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I just fixed a similar problem that was using a wireless keyboard the batteries were dead.

The original key board was also messed up. so I brought one of my working keyboards to the friends house and it worked just fine on the machine.

I also hooked up an exteranl drive with an operating system on it via firewire Held down the option key and when it started up it showed me the external drive I then booted with that Opearting system which had been restored to the external drive form a different machine, Then I ran a defrag program which said the drive was 64 % fragmented. So there was more than one problem wrong. I defragged the drive using drive genius and replaced the batteries in the key board and all is well

Apple users think that defragging is not needed they are sooooo wrong about this thought.

Mac cannot defrag themselves with MAC OS software It requires third party software

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Got the same problem. iMac is booting properly, but the screen gets white after Apple logo and spinning circle. I tried booting from external drive and system dvd and usb without success. But I get access to the imac's HD via network. Is this a hardware failure or could it be a software problem? Did steve or mayer get this resolved (years after)?

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