MacBook Air A2179 Not turning on, drawing 0.03-0.06A at 20V repair

Problem description
No Power, 20v and ~0.03-0.06A current draw on the 820-01987 logic board diagnosis and repair.

Symptoms

 * MacBook not turning on
 * Pulling ~0.03-0.06A current at 20v from USB-C as measured by USBC meter

Check for DFU or Recovery Mode

 * Connect the MacBook Pro A2179 to another Mac or MacBook via the master port (bottom left side USB-C port).
 * Use Apple Configurator 2 to verify whether the device is in DFU or recovery mode.
 * If the device is in DFU mode, proceed to "Device stuck in DFU mode due to corrupt T2 firmware" in the repair steps below.

Check for a short to ground on PP3v3_S5
If a short to ground is found, proceed to the "PP3v3_S5 short to ground" repair steps below.

Device stuck in DFU mode due to corrupt T2 firmware

 * Revive firmware via Apple Configurator 2.
 * Ensure the MacBook is running the latest macOS version for consistent results. Check for MacOS updates prior to reviving/restoring T2 firmware.
 * Follow the provided Apple support article for the procedure.
 * Plug the device you are working on to another Mac or MacBook via its master port. The master port on the A2289 is the bottom left side USB-C port (closest to the trackpad).
 * Once plugged in, open Apple Configurator 2. You should see a big square icon pop up that says "DFU" or rarely, "RECOVERY".
 * Click the icon, Navigate to the top menu bar click "Actions" then "Advanced".
 * Select Revive device. You will see a progress bar appear. This process can take over 30 minutes in some cases.
 * Be cautious that selecting "Restore" will wipe all user data.

PP3v3_S5 shorted to ground
How to find short circuits


 * Locate and replace the shorted component.
 * If PP3v3_S5 is shorted to ground, pin 5 of U4885 is a good place to inject voltage to.
 * 1v at 5 amps is an appropriate voltage to inject into the line.
 * With voltage being injected, perform thermal imaging of the board. If thermal imaging is not available, feel around the board to see where it is getting warm. Once the area is localized, add a small amount of isopropyl alcohol to the area to localize the shorted component.
 * Once the shorted component is localized, replace the shorted component.

Forcing a firmware revive

 * If no short is found on PP3v3_S5, and the device is not in DFU or recovery mode, you can try forcing a firmware revive.
 * Attempt to restore BridgeOs firmware via Apple Configurator 2 by placing the device into DFU mode.
 * STOP! Before you begin, is your Mac on the LATEST VERSION of MacOs? If not, update your system before proceeding. Forcing a MacBook into DFU mode, and attempting to restore BridgeOs firmware on a old version of MacOs may result in a bricked device.
 * Selecting restore will wipe all user data.
 * Follow the instructions on this Apple support article on how to revive or restore T2/BridgeOs firmware, including on how to force a Intel based MacBook into DFU mode by using a key combination.

If the device fails the firmware revive or restore, the T2 chip or one of the NANDs is likely dead or is receiving unstable power from the PMIC (U7800) causing it to crash. If the device fails the firmware revive, you can try empirically replacing U7800.

Replacing U7800 should be seen as a last ditch effort, as it only works in a small amount of cases, and does not have a definitive diagnostic test to determine if it is bad.