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I have a MacBook Pro (mid-2009) that I just installed Mountain Lion on. After the install everything appeared ok, but a day later it goes straight into sleep/hibernate whenever I turn it on.

This happens even when I press Command+R during startup - before it gets to the menu screen it hibernates.

What can I try to fix this?

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Before doing an SMC Reset as suggested by @Thecafremo, I decided to clear the PRAM/NVRAM (because the SMC Reset page says: "an SMC reset should only be attempted after all other standard troubleshooting has been performed")

This seems to have been enough to bring the MacBook back to life at least for the moment.

Clearing the PRAM/NVRAM is achieved by holding down Command+Option+P+R during boot before the gray screen appears and letting the startup go thru three chimes before releasing the keys.

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    And letting the startup go thru three chimes before releasing keys. – Zo219 Apr 28 '14 at 8:06
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Usually a SMC Reset is the first thing to do in this kind of situations. As it manages Fans, Lights and Power. To do so, being your MBP an Unibody, if I'm not mistaken, just:

  1. Shut down the computer.
  2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
  3. On the built-in keyboard, press the (left side) Shift+Control+Option and the power button at the same time.
  4. Release all the keys and the power button at the same time.
  5. Press the power button to turn on the computer.

Note: The LED on the MagSafe power adapter may change states or temporarily turn off when you reset the SMC.

  • it is unibody - I'll try this when I get home, thanks – user27924 Aug 24 '12 at 12:30
  • I was cautious and cleared the NVRAM first as the page you linked said "an SMC reset should only be attempted after all other standard troubleshooting has been performed". Clearing the NVRAM seems to have cleared the problem for the time being but I'll bear this in mind if it comes back again. – user27924 Aug 26 '12 at 17:54
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I also found the reed sensor was bad in my 2009 13" MacbookPro. I hooked it to an external monitor & the computer stayed awake & functioned normally. Just the laptop with the lid open - instant sleep on power up. The 'top case' has to be replaced.

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Found the lid sensor had gone faulty on one I was fixing. (Booted Ubuntu, turned off 'sleep when lid closed'.

  • Can you please add some details about how you diagnosed the issue? The Ubuntu reference is a bit hard to understand in the current form. – nohillside Jul 3 '13 at 9:38

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