Chosen Solution
I recently spilled half-cup of tea to my macbook. After drying it I turned it on and found out that left shift key was always in pressed position. I tried to unmount the keys and clean under them but it had no effect. So I ordered a new keyboard and replaced it on my own (was doing it first time in my life). Everything seemed to be ok, after a short period of time left ctrl stopped working. Of course I was upset and thought that’s my fault and I didn’t do replacement work good enough. But after some time left shift become stuck again (not the button itself but it’s software status). And now I’m confused, how the same keys could be broken again on the completely new keyboard? I’ve changed the board itself all the keys with pressing mechanisms, wires going to motherboard. The only original part is backlit layer. How can that happen? Maybe I should try to plug and unplug the keyboard wires from the motherboard?
It could be the keyboard backlight; if there is dried liquid on this, it can heat up and effect the keyboard membrane. As strange as it sounds, I have had a MacBook returned due to this issue twice. I have never seen a keyboard go faulty (unless it has had contact with liquid), so for it to happen twice on the same MacBook, I was confused and thought it could maybe be a bad batch of keyboards. It was only when I noticed there was a bit of liquid on the backlight, underneath the CPU area, I realised it would have heated up (becoming wet) and would damage the new keyboard. Replace the keyboard and backlight, should resolve your issue (unless it is logic board related, but you say you have replaced the board so this should be fine). Here’s how to do it: MacBook Pro 13" Retina Display Late 2013 Keyboard Replacement
