A number of users are reporting that MacBooks previously susceptible to a sudden shutdown when performing the dev/null test (entering the Terminal command, yes >/dev/null & that indirectly triggers high temperatures) are now immune.

MacFixIt reader Rick Borie writes:

"I have a MacBook that would shut down but rarely. When I ran the test listed on one of your pages ‘yes >/dev/null &’ it would shutdown pretty consistently. I just installed the firmware update and have been running the test as well as least 6+ apps and it hasn’t shutdown yet.’

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Apple has released the MacBook SMC Firmware Update 1.1, which is intended to resolve a widely reported issue where MacBooks suddenly shut down during normal operation. According to Apple, the update "improves the MacBook’s internal monitoring system." It is recommended for all MacBooks, even those that have already undergone warranty repairs for the sudden shutdown issue.

This is an issue we’ve covered extensively on MacFixIt:

• Repair, delays waiting for parts; Parts being replaced: Heat sink, Logic board; more
• More sudden shutdown commentary; faulty temperature sensors
• Possible hardware fix for sudden shutdowns
• Sudden shutdowns — possible fixes
• More on sudden shutdowns
• MacBook (13"): Suddenly shutting down

You must have Mac OS X 10.4.8 installed to see this item in Software Update. However, it can be installed under Mac OS X 10.4.7 or later. 

Like many other firmware updates and other Apple updater utilities, this download will place an application in the /Applications/Utilities folder.

Apple’s instructions for installing the update are as follows:

1. Make sure the power cord is connected, and power is flowing.
2. Quit all other open applications.
3. Launch the MacBook SMC Firmware Update application (locaed in Applications/Utilities), then click Restart in the MacBook SMC Firmware Update window and wait for your computer to restart.
4. The SMC firmware update starts automatically. A status bar indicates the progress of the update. During the update your computer fans will run at full speed, but will return to normal once the update completes.

The update is available as a sub-1MB standalone download.

 More Information In http://www.macfixit.com