Molar tooth with crown has very obvious fistula pus tract where tooth and gum meet. No tooth pain. Not sensitive to hot/cold. See an endodontist or?

Poor prognosis. Poor prognosis. See DDS first. He will refer if indicated.
Two options. 1) See an Endodontist for best advice as to prognosis of root canal treatment, or 2) see your General Dentist for advice as to root canal vrs extraction. Either choice will provide you with good advice.
Fistula.... I personally advise you first see a dentist to discuss your options based upon a clinical exam and x-rays. You should understand the pros and cons of each option and prognosis of Endodontic treatment followed by restorative treatment. Your General Dentist may then elect to treat you or refer you to an Endodontist (or Oral Surgeon).
Cause? Endodontic or periodontal in nature? Each condition will require a different approach. See your dentist first.