This is what the Apple Store did for me.
Reset the System Management Controller (SMC):
- Shut down the computer.
- Plug in the MagSafe or USB-C power adapter to a power source and to your computer.
- On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
- Release all the keys and the power button at the same time.
- Press the power button to turn on the computer.
- On MagSafe power adapters, the LED might change states or temporarily turn off when you reset the SMC.
For more information: https://support.apple.com/en-us/HT201295