I recently went through and replaced the left joycon rail. Everything seemed to be working fine, until i started playing it on its dock. During gameplay, it errored, saying it was overheating and needed to go into sleep mode. When i undocked it, the device was very hot. After letting it cool down, i played it while connected to a charger, and it never overheated. I tried a new dock, just to be safe, but the same issue occured there as well. I assume I may have incorrectly reassembled it and something is wrong when it tries to push the display through the dock, causing it to overheat. Any ideas?
I’d have a look if the fan is working in the first step. If yes, it could be helpful to replace the thermal paste.
If fan is working and thermal paste is changed it should not overheat again so quickly.
Thank you for the comment! I initially did that, but it didnt help. Im emberresed to admit what it was, but it looks like i just missed the ribbon cable for the fan 
. I opened it again to replace the other rail, and noticed it was just sitting on top of the connector, not pushed in lol thanks again tho, nice to know i jave a community to lend a hand when needed!