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Some weeks ago, I went to a client's site to install a new router (Macs rock for IT work!) and when I opened my MacBook Pro (5,1) it didn't return from sleep; the screen stayed black and the indicator light glowed a steady white. I opened and closed it a couple times, but still nothing. I presumed some fault and so I hard-powered-off. When I powered back on, I got the S-O-S chimes described here (I know that article is for the Air). This continued several more times. I returned to the office and I did some searching and people recommended Cmd-Opt-P-R to reset PRAM, which I tried to no avail, until...I was pressing random keys on the keyboard and it started right up.

I had to swing by the Apple Store for another user, so I asked one of the Geniuses there what his experience was, and he told me that SOS beeping is the logic board, and that pretty much there's no hope. I was intrigued, and did not tell him that it was sitting on my desk, running fine, at that very moment.

Fast-forward a couple weeks, and my Mac works perfectly fine, except when it needs to reboot. I find that if I need to reboot, I have to clear the PRAM and then press S or C to start the computer. Sometimes it takes more than one try. It will sleep and wake perfectly every time (usually 2x per day, with car rides) and will never give a single issue while running. Not a hiccup.

Someone suggested a bad solder joint which is being affected by temperature, but I think that's not the issue: I can leave the computer in the cold car for an hour, sleeping. It wakes up completely fine. There's not enough current to be heating any part of the board to maintain contact on a failing joint. Similarly, I can finish some photo editing and the system is hot and I can close the lid, put it to sleep, and remember 'one more thing' (It's an Apple thing, what can I say?) open the lid, and it resumes flawlessly.

Everything I can find on the SOS chimes is 'Hope you got AppleCare, that means your logic board is gone, man' but that is (in this case) demonstrably not true. Has anyone else experienced anything like this? Is there anything else I can or should try? This has all the symptoms of a firmware glitch to me, or some strange EFI issue. No one else, apparently is experiencing it at this time. Anyone?

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I've heard of a logic board "limbo state" where it passes in and out of working. It is broken, but it's letting you use it until it decides to die altogether.
You may as well get all the life you can out of it, but don't depend on it to heavily.

I can't find confirmation of this, but I know I've heard stories of it before. Can anyone confirm?

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"it's letting you use it until it decides to die altogether". Gasp! Don't you understand computers? No!, it's waiting until a totally critical moment when you absolutely have to have it work, like just before backing up all your data before taking it in for a repair. –  Greg Jan 20 '11 at 20:02
    
Well, I had all the backups up to date. But ironically enough, it died entirely after this post. I know it's dead, too, because I waited until I had another (brand new) MacBook Pro at my desk, and tried to turn it on. If it could have, I know it would have started up then. –  atroon Jan 21 '11 at 0:24

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