MacBook Air turns off whenever lid is closed after High Sierra update

Solution 1:

The first thing I would try is resetting the System Management Controller.

Reset the SMC

On your particular Mac, the steps to follow are:

1. Shut down your MacBook Air
2. Unplug the power cable from your MacBook Air
3. For 10 seconds, press and hold at the same time the shiftcontroloption keys (on the left side of the built-in keyboard) and the power button
4. After 10 seconds let go of all keys and the power button
5. Plug in the power cable
6. Turn your MacBook Air back on with the power button.

Once you've done this, test your MBA by using it as normal to see whether this was enough to rectify it.

Solution 2:

The first thing to do it probably identifies the cause of the shutdown. To do it:

1. run log show --predicate 'eventMessage contains "Previous shutdown cause"' --last 24h

2. understand what's the cause of the shutdown (list available here)

3. take appropriate action ;)