View source for MacBook Air A2179 Not turning on, either 5V or 20V on USBC but with low current draw repair

{{Repair Guide
|Device=MacBook Air A2179
|Affects parts=Motherboard
|Needs equipment=multimeter, soldering iron, soldering station, thermal camera
|Type=Soldering
|Difficulty=3. Hard
}}
{{stub}}
==Problem description==
When connected to the USB-C meter, the 820-01958 board draws very little current at either 5V or 20V. Not turning on and SLPS2R and AWAKE voltages/signals are cycling.
[[File:Placeholder image.jpg|thumb|Example image (Figure 1) -- No image yet. Help expand this page by uploading it!]]
==Symptoms==

*MacBook A2179 not turning on.
*Little to no current draw as measured with a USBC ammeter. 
*when measured, SLPS2R and AWAKE voltages/signals are cycling.

==Solution==
===Diagnostic Steps===

==== Check voltage and resistance to ground on PP5v_G3S ====

* When measured while the board is off, the resistance to ground on PP5v_G3S is typically large, well within the kilo-ohms.
** If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
* When the voltage is measured while the board is on, you should expect 5v. 
** Values less than 4.9v should be considered abnormal. If that's the case, see the relevant repair steps below

==== Check voltage and resistance to ground on PP3v3_G3S ====

* When measured while the board is off, the resistance to ground on PP3v3_G3S is typically large, well within the kilo-ohms.
** If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
* When the voltage is measured while the board is on, you should expect 3.3v. 
** Values less than 3.2v should be considered abnormal. If that's the case, see the relevant repair steps below

==== Check voltage and resistance to ground on PP1v8_G3S ====

* When measured while the board is off, the resistance to ground on PP1v8_G3S is typically large, well within the kilo-ohms.
** If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
* When the voltage is measured while the board is on, you should expect 1.8v. 
** Values less than 1.7v should be considered abnormal. If that's the case, see the relevant repair steps below

==== Check voltage and resistance to ground on PP3v_G3H ====

* When measured while the board is off, the resistance to ground on PP3v_G3S is typically large, well within the kilo-ohms.
** If you measure less than 100 ohms, then you most likely have a short on this rail. See repair steps below.
* When the voltage is measured while the board is on, you should expect 3v. 
** Values less than 2.9v should be considered abnormal. If that's the case, see the relevant repair steps below

===Repair Steps===

==== Short circuit on one of the rails or less than 100 ohms measured ====
[[How to find short circuits]]

* Inject 1V 5A to the shorted rail.
** With either a thermal camera or your hand feel for the area that is heating up.
** Identify and replace the shorted component.

==== No short is measured but voltage is still missing ====
Replace U7800.