Let's go over another Macbook Repair here - an A1466 that is not turning on. This Macbook Air is giving us a green light on the charger, which tells us that PP3V42_G3H is present. PP3V42_G3H is what allows the onewire circuit to turn on, so the light can turn on.
Since it wasn't turning on, we decided to go down the list of power rails, as listed in the schematic. We found that the machine had PPBUS_G3H on one side of its fuse, but not on the other. We found a short to ground on the side of the fuse that had no power, and saw that the fuse itself was blown. Sad.
We looked around and saw that a capacitor on PPBUS_S5_HS_COMPUTING_ISNS looked wrecked. We scrolled through the schematic and saw that this rail comes from PPBUS_G3H. It goes through a current sensing resistor whose purpose is to be part of a current sensing circuit that will allow the machine to tell how much current every component on PPBUS_S5_HS_COMPUTING_ISNS is using. This explains how a short circuit on PPBUS_S5_HS_COMPUTING_ISNS can affect PPBUS_G3H.
We removed the ailing cap, clearing the short circuit. Now, the fuse is ready to be replaced. Replacing the fuse would do nothing, as it would simply blow again as a result of the cap short to ground. After replacing both, the machine turns on and takes a healthy 500+ mA - fanspin has been achieved.