The symptom
The MacBook wouldn't turn on, no signs of life at all. When the MagSafe charger was plugged in, no LED would light up. The power draw was 5V 0.05A.
The diagnosis
First, some basic tests were carried out:
Trying a different charger, swapping the MagSafe connector, disconnecting the battery and peripherals, but none of this was enough.
The logic board was then removed from the chassis for more precise voltage measurements needed for the Mac to boot. The 3V8_AON line was absent.
What was done
The main power rail (PPBUS_AON) on the board was present, but not the 3V8_AON, which is generated by U5700. We therefore turned our attention to this chip, which became the main suspect. But before that, it was necessary to check that it was actually able to do its job, meaning properly powered and receiving its enable signal, which tells it to become active.
In our case, the enable signal was low (2.20V instead of 4.50V). After further analysis, a problem was found in the circuit generating this signal: a capacitor was cracked and was short-circuiting the line.
After replacement, the 3V8_AON line was operational again.
The Mac was reassembled and booted up fine. A relief for the client!
But surprise... after a few tests, the technician noticed that the left speaker wasn't working, only a crackling sound came out of it. After replacing the speaker, the problem persisted.
The issue actually came from the logic board: one of the two chips (UR600) that drives this speaker was dead.
After replacement, the speaker worked again.
Outcome
After resolving these two issues on the logic board, replacing a capacitor and a chip, the client's MacBook Air M2 was working perfectly again. All the data and the macOS session were preserved, as if there had never been a problem.
Another satisfied customer!