My 2015 Air shut down suddenly two weeks ago. The battery was at about 80%. When I got it home and plugged it in it showed classic bad SMC behavior — the power diode didn’t light.
I did an SMC reset and it worked, but a week later it did the same thing. I did an SMC reset again, but without checking if it was necessary.
It happened yet again. This time it worked fine as a soon as I plugged it in. That gave me hope that it was a battery issue, even though system info showed only 80 or so cycles. It’s an old battery.
After doing the usual 3 backups-to-current-state-prior-to-repair (one update to my Carbon Copy non-bootable clone backup, one fresh full bootable clone, and one Time Machine backup) I brought it in. It failed the diagnostic test with a big red dead battery note.
So $140 when the part comes in, which is a nice relief. If it had been the motherboard that would be $340 and I’d have a machine with a 4yo battery and a 4yo SSD. Might be better to just buy new.