MacBook shutdowns during boot up, with an unusual problem and fix.
My Macbook Air 13 (early 2014) started rebooting during startup with no apparent pattern. I was able to complete a login just once.
- SMC reset, PRAM reset had no effect.
- Re-seating SSD had no effect.
I suspected a logic board or RAM failure as there was no pattern to the reboots.
Came across a post on another forum where the user described a keyboard failure on their Pro. They were also unable to use any of the startup key sequences to boot from an external drive or run hardware diagnostics. Their fix was to replace the keyboard.
I decided to clean the keyboard by rubbing every key in all four directions.
Whatever was choking the keyboard got reset or dislodged and the system booted up as normal. It seems a stuck key can overload the keyboard microcontroller and trigger a crash/reboot.
Leaving this out here in case someone else has this strange problem.