Is the inability to complete a yawn a symptom of tmj disorder?

Mobility Limited?: If this is related to limited movement of the jaw, or inability to get the full range of motion of the jaw, then yes!

Possibly: If you can't complete a yawn because you cannot open wide, that can be a symptom of a TMj disorder.

Definitely, yes: The pain of myospasm from TMJ can vary from a dull ache to intense pain with movement. The acutely shortened muscle may result in a markedly reduced range of motion and prevent complete yawning. See Orofacial Pain specialist for evaluation and treatment.