2008 iMac won't boot, gets restart message

That can be a fair number of things. I doubt it's the drive or drive related, but in case it is, the following article has information about some of the resets you can do with a Mac, as well as other problems that are drive related:

http://scsc-online.com/Bad Hard Drive Signs and Symptoms.html

It sounds like you tried most if not all of the resets, but in any case it may help you. The article has a lot of stuff about bad hard drives and the types of problems that are hard drive related and those that act like they're hard drive problems when they aren't. It might help you, but I doubt the problem is the drive.

My guess, honestly, would be the logic board or a problem with something plugged into the logic board. Poorly seated Airport cards can sometimes cause a system to throw exceptions during startup. It could be the video card, but I would expect to see some type of strange artifact in the video if it was, or even no video at all. Bad RAM or poorly seated RAM can cause the problem as well. I suppose anything plugged directly into the logic board could potentially cause problems if it isn't seated properly. Of course the logic board itself might be bad as well, but I wouldn't assume that without checking other simple stuff out first. On some of the old PPC laptops the Airport card could wiggle itself loose and the system would lock up during boot.

It could be drive related (bad cable, bad drive, corrupt boot files in the drive) but I doubt that because you said you tried booting it from a USB w/Lion and in target mode from another system. If any of the drive related problems I've just mentioned were present, booting from external media should have succeeded by bypassing the internal drive and cable, but it didn't, which points right at the logic board or something connected to it.

I would try and install from the boot media just to see if it works. If you have a copy of AHT available, you might be able to get it to run and it might be able to tell you the problem.

Hi. I've got a 24" Early 2008 iMac with the ATI Radeon HD 2600 Pro graphics card.

I seem to remember reading that the nvidia cards in these Early 2008 iMacs were very very likely to fail, and that graphics card failures can prevent boot up (though I don't know about your particular symtoms).

I'll admit that I'm too lazy at the moment to look it up and produce supporting evidence, but you might want to look into this hypothesis...

Hope you figure it out because it's still a good machine -- I still use mine as my main daily use machine (although I've added an SSD which helps).

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...wanted to reinforce FrtzPeter's suggestion to run the Apple Hardware Test. If yours is an Early 2008 iMac, it should be on a gray-with-white-lettering disc that came with the computer, labelled "Mac OS X Install Disc 1". (Don't know if yours is 17" or 24" but I'm assuming it'd be the same as my 24".)

If you have nvidia graphics (8600GT) that might be your issue. But I would verify see if you get any errors with ram.

2008 iMac won't boot, gets restart message

I believe that if the GPU failed, it would be no display at all.

Is it always the restart screen that show up? I would try removing 1 of the 2 ram and try again.

To test the ram... There is a small utility memtest86 to test ram.

I've had a number of these come through where I work. And 99% of the time it IS the video card.

I have had success using a heat gun to reflow the solder (what fails in the 8XXX series). But it's only a temporary fix. This also applies to 2007-2008 MBP's with the 8600m.

There is a more permanent fix that involves swapping the failed Nvidia card with an ATI card that shipped in the lower end model, that is less powerful but much less prone to failure.

There is a thread here explaining the process and showing the part numbers.

twistedbydesign said:

I replaced my Nvidia 8800 GS with an ATI Radeon HD 2600 Pro. The product number for the ATI card is 661-4663. This card was a direct drop in for the Nvidia Card.

Click to expand...

For the cost it may be worth it. I've seen them around 100-200 depending on the source. The 2008 iMac is certainly still capable, I've got a Mid 2007 20" kicking around with 4GB RAM and an SSD running Mavericks and Yosemite perfectly. Best of luck!

LovemyVolvo said:

Thanks BrettApple....

I looked at the diagnostic lights and all of them are working. I'd like to know how you used the heat gun to "fix" the solder.

I've found the card ranging fom 365.00 to over $500. If I can try resoldering it to see if my problem resolves, then I'll feel more comfortable spending the $$$ to replace.

If for some reason it isn't the card, I want to be sure before I put that money out just to find out is the logic board:-(.

Click to expand...

Essentially what you are doing is heating the solder up to it's melting point to make it flow back into the chip and attempt to get rid of any cracks that are keeping it from having proper contact between the chip and the board.

Defects like this don't always show up in test like AHT, most of the iMacs I've had with the issue will either pass AHT, or just freeze part way though or kernel panic. Even if the logic board and RAM themselves are perfectly fine. I've tested with an old ATI card from a 2007 iMac to check the rest of the hardware.

It is an involved process though. Removing the video card is the longest portion of it. I just used iFixit's guides on replacing the video card on other 24" or 20" iMac's and they're close enough to get the card out.

Then you remove the heatsink and thermal paste, wrap foil around the card and cut out a hole for the chip itself (this helps apply the heat where it needs to go and shield the rest of the board). Then I start on low and slowly increase the heat to the chip till it gets to it's melting point, then slowly back the heat back down till it's on low, then let it cool off for a good long while. Then reapply new thermal paste, and the heatsink, and put it back in. And bingo, good as new for anywhere from a few hours to a year or longer. Never know how long it'll last, but it's usually good enough to use while you consider your options.

A few pics from a Dell Vostro 1400 I recently repaired with an Nvidia 8400m, much like the 8600m in the 2007-2008 MBP and your 8800 in the iMac. All have the same defects. It was previously either artifacting or not booting at all.

BrettApple said:

Essentially what you are doing is heating the solder up to it's melting point to make it flow back into the chip and attempt to get rid of any cracks that are keeping it from having proper contact between the chip and the board.

Defects like this don't always show up in test like AHT, most of the iMacs I've had with the issue will either pass AHT, or just freeze part way though or kernel panic. Even if the logic board and RAM themselves are perfectly fine. I've tested with an old ATI card from a 2007 iMac to check the rest of the hardware.

It is an involved process though...

Click to expand...

Thanks very much BrettApple for your postings. Very informative, and your apparent experience lends weight to your statements. This could be very valuable information for others having trouble.

I'm lucky enough to have the ATI Radeon HD 2600 Pro graphics card. I have had the "screen freeze" problem with this card (resolved by installing old versions of the ATI kext drivers), but it has mysteriously disappeared under Mountain Lion. So LovemyVolvo, if you determine that the nvidia card is the problem, I'd reinforce BrettApple's suggestion of looking for one of these to put it. The thread he linked to looks quite useful.

There's an easy way to check if the graphics card is at fault - pull the card. OS X does not need a graphics card to boot.

Take the card out, and boot the machine headless. If it boots ok (you can verify this by something such as volume control, remoting in, pinging its IP, etc, then the graphics card is at fault.