iMac suddenly shutdown with previous cause -128

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Previous shutdown cause: -128

A negative shutdown code may indicate a hardware problem. The precise meaning of the codes is not publicly documented.

If you haven't already done so, you can try resetting the System Management Controller. Otherwise, or if the reset has no effect, see below.

Make a "Genius" appointment at an Apple Store, or go to another authorized service provider. You may have to leave the machine there for several days.

Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keeping your confidential data secure during hardware repair

Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.

I don't know how to reset SM.

The instructions are linked to my last comment.

iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program - Apple Support

Any new updates? I am curious because I have the same problem.

Sorry to hear that. I am still trying to figure something out. Until then, I will have to run in Safe Mode.

Hi Yukka.

I had the same problem with my iMac 9,1 early 2009 with Nvidia geforce GT 130 512 MB. Even though Apple doesn't publicly post what the previous shutdown code means, I'm positive that previous shutdown cause -128 is a failure of the graphics system. Now most people think it's the graphics card alone, but they forget about the pci-e slot. The first go around, I reflowed my graphics card in the oven. Go to this thread and the last post for the explanation of what I did Re: Early iMac 2009 shuts down unexpectedly Yosemite only .

The fix lasted for about a month and a half before the -128 curse started reappearing sporadically. So I reflowed the card again at a higher temp and longer time (415 F for 15 minutes), having positioned the card lower to the tray (about .5 cm clearance between card and foiled tray) to make sure the underside was getting good heat. I also cleaned the card thoroughly after the bake and cooling down...then reapplied new thermal paste (the pads where still good).

The only difference this time was that I inspected the inside of the pci-e slot. I began scraping the connectors with a narrow plastic slice from an expired credit card (if I you have a very small, wire brush, that would be better). A lot of gunk started coming up. So I thoroughly cleaned the slot, scraping then going over it with a cotton swab with alcohol. I then reinstalled the card and I haven't had a problem since. It makes sense because the pci-e slot supplies power and if there is gunk on the connectors, that will interfere with certain power functions.

So what's the upshot? It could be a problem with both the graphics card AND the logic board (pci-e slot)!

I can tell you that your problem will not get better, only worse, without intervention. Operating your computer in Safe Mode is probably not cutting it for you. Now, there are risks involved in reflowing your graphics card and it could ruin it, but if you want to get your computer back, fully-functional, I would recommend the procedure.