Last night, I came home after a long day and found that my MacBook Pro would not wake from sleep after I took it out of my backpack. This is an occasional problem with my aging notebook, but after a reboot, it usually operates as it should. But this time, after I rebooted the machine, the fan spun to full speed, making that disconcerting whirring sound, despite the fact that the computer was not running any applications and was cool to the touch.
I opened up the computer and found that the outermost RAM DIMM had popped out of the slot. That would explain why, after letting it boot earlier, it reported only 4 GB of RAM. After reseating the RAM and securing each connection, I rebooted the machine again, but the fan kept spinning at maximum speed. Fuck!
This computer has been through a lot. I not only spilled seltzer on it, followed by black coffee. Eventually, the liquids damaged the keyboard, which I had to replace. And, let’s not forget about the time I had to solder the cable connecting the fan to the logic board. I suspected that whatever dislodged my RAM must have damaged the temperature sensor, and I dreaded that it would finally be beyond repair.
But then I noticed that MagSafe Power adapter, which was connected to my computer, was not illuminated—whereas it is always either green or amber. Moreover, the battery status lights also did not illuminate to show the charge level. Those additional symptoms led me to the underlying problem. The System Management Controller was corrupted, and I needed to reset it.
I’m old enough now to remember when our Macs would develop unexplainable problems, someone would advise us to zap the PRAM. In my over twenty years of working with Macs, zapping the PRAM never once fixed any mysterious problems. Would resetting the SMC do any good?
The Apple Support page outlines the steps for resetting the SMC for particular kinds of computer, such as portables with removable batteries, portables with sealed batteries, and desktop computers. In my case, I needed to shut down the computer, hold down Shift–Control–Option with my left hand and depress the power button with my right hand at the same time. Once I did that, the lights on the MagSafe adapter came on.
Once again, I rebooted up the computer with baited breath. This time, as it had done countless other times, the fan came on at the normal speed.
While zapping the PRAM was for me pretty ineffective, resetting the SMC had a different result. It did indeed fix something!