Chosen Solution
My A1405 MBA has been somewhat neglected for a while and I recently went to use it and found that, when powering it on, the CPU fan would slowly start spinning faster and faster but nothing else would happen. No boot sound, no keyboard lights, nothing. I’ve tried resetting the NVRAM, PRAM and the SMC. There is no audible feedback from the Macbook when doing so. Thinking it was the battery, I removed the rear cover and disconnected it, plugged in the charger and had the same experience. Is she dead? Is it worth trying to replace the logic board? It’s a 4Gb RAM, 1.5GHz i5 so it’s not much to write home about but I still love it dearly!
One last shot - disconnect the battery and mag-safe. Press and hold the power on button for 10 seconds. Reconnect the battery and mag-safe, you should get a spontaneous reboot. The logic board can be cleaned. If it came into my shop, I would clean off the corrosion with pure alcohol and drop it in the ultrasonic. UPDATE Electronics Water Damage
Try reseating the I/O board cable and also try booting without the SSD and see if that lets the MacBook turn on and make a chime sound. MacBook Air 13" Mid 2012 I/O Board Cable Replacement MacBook Air 13" Mid 2012 Solid-State Drive Replacement
