I am working on a custom merchant. I gave him only one AI package which is to sit in a specific chair all the time. When I loaded my mod to test it, he is in the right spot, but he yawns constantly. I'm talking like every two seconds.

I tried Google but couldn't find anything helpful.

That's the default idle animation for a seated NPC and will run as long as he is not instructed to run any other animation while seated. In his AI, tick No Idle Anims, should stop him.