The first step to diagnosing a MacBook is to go through all the power rails required for the machine to turn on. Since this MacBook has a light on the charger, we know that our PP3V42_G3H rail is present. Next up is PPBUS_G3H. We measure this to be 4.1V. This rail should be somewhere around 12V. This is most likely why our machine isn't turning on. The first place we look is our U7000 chip, which is the ISL6259. This chip is responsible for creating PPBUS_G3H. Next to U7000, there are some resistors that allow the ISL to sense current. After checking these resistors, we find that they measure OK. However, around the ISL chip we find a corroded via on the charger cell line. This is required for PPBUS to have 12V. CHGR_CELL is going to determine how many "cells" are in the battery - does the chip produce 8.16v, or 12.26v?
How do we fix the issue?
We fix the broken trace with a small micro-jumper across the corroded area. After touching up all the other solder joints and resistors with fresh leaded solder, we measure 12.59V on PPBUS_G3H. Now, the machine has power as it should.