Chosen Solution
My MBA stopped turning on. Doing an SMC reset I can see that the power button works (the led on the magsafe flashes green), and it is charging because after a while the green light stays on solid. But no response when I try turning it on. I’m wondering if this could be an issue with the lid (hall effect?) sensor or if it’s likely to be something more serious? Any advice greatly appreciated
Unplug the battery, I/O cable (cable which goes over fan) and trackpad cable. After doing this, plug in the charger. If it powers on, process of elimination to find out whether it is the I/O cable, I/O board, battery, trackpad cable, trackpad or keyboard shorting a signal to ground. Check the back and front of the logic board for liquid damage. Chances are there is liquid around the SMC chip preventing it from booting, this is very common in 13" Airs. Clean with 95%+ isopropyl and if not successful, the board will require diagnosis. At this point, if you don’t know how to microsolder, it is best sending it to a MacBook repair shop.
In addition to Reece’s answer try a PRAM reset: Here’s how to reset your PRAM: Shut down your machine. Yes, all the way down, not sleep or logging out. Press the power button and then press command-option-p-r. … Hold those keys down until your Mac reboots again and you here the startup chime. Let go of the keys and let your Mac reboot normally. Apple also has a “rate Rate Depot Repair” Total with tax and shipping $357. Total time 4 days. Shipping is FedEx overnight and they send you the shipping box. This is if it does not have impact or water damage.
