Earlier today, I replaced the old, worn out battery in my MacBook Air to a newer one.

Upon startup, the fans within the computer went off at full speed and the entire computer is laggy. I have attempted to restore my computer from a Time Machine backup by holding both Command+R and Option (two different startup times) to which both have arrived at the current result I'm at:

My MacBook Air does not go past the initial startup screen and is stuck with a 0% progress bar.

I've attempted to restart numerous times and I'm fairly certain this isn't helping anything. I assume that the high fan usage is due to the CPU being overworked, but what would be using the CPU that much on a startup?

My question is, how do I go about fixing this? Attempting to enter the macOS Utilities causes a road block before I even start to progress there.


  • MacBook Air
  • Mid 2012
  • latest public macOS release
  • 1
    Did you perform a battery calibration? If not, do that. If you did, try resetting the SMC.
    – NoahL
    Jan 14, 2017 at 2:10
  • @NoahL I cannot do a battery calibration at the moment, running into problems of just utilizing the computer for more than a paperweight. Did the SMC reset as stated from this discussion and the lag has seemed to dissipate. The fan speed is still high but not maxed. Jan 14, 2017 at 2:20
  • Is the computer warm to the touch? Feel above keyboard at base of hinge with screen. Does it feel cooler than the air, slightly warmer, or hot (warmer than comfortable)? If you can, try doing a clean OS install by booting into recovery and reinstalling the OS without restoring from Time Machine.
    – NoahL
    Jan 14, 2017 at 2:21
  • Do you still have the old battery? If so, try putting that back in and while plugged into the charger will it start up fine or behave different than it does with the new battery?
    – Dan V
    Jan 14, 2017 at 2:23
  • 1
    Try resetting the pram. On power up immediately hold the command option P and R keys till you hear 2 or 3 chimes and screen flashes. Otherwise it seems to be acting like a cooked CPU. Has it been cleaned out internally and/or around a dusty or smoke environment?
    – Dan V
    Jan 14, 2017 at 2:39

1 Answer 1


The third-party battery was too powerful, despite being the same model number (A1466).

According to the Mac Resource Inspector at the Apple Genius Bar, the logic board was overcharged. This made the fan run at a minimum of 6200 rpm and subsequently forced kernel_task to go insane.

  • Replacing the logic board and original Apple battery was valued at roughly a minimum of $675. According to the Apple Genius Bar Technician, it was a 50/50 guarantee, because everything touches the logic board and those pieces could be faulty as well, therefore costing more money. Since the MacBook Air was purchased for $800, a new MacBook was purchased instead. Jan 24, 2017 at 0:43

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