A 23 year-old female presents with swelling and constant pain related to the first right mandibular molar. Previous root canal retreatment was completed 3 months prior. A linmited field-of-view CBCT was taken for further diagnosis and treatment planning. The scan demonstrated a large apical and furcation lucency. Root canal retreatment was recommended and completed. A mid-mesial canal was found untreated. All canals were cleaned and disinfected using negative pressure and passive ultrasonic irrigation. On the final post-operatory radiographs, the portal of exit of the mid-mesial canal leads to a mid-root area correlated with the furcation lesion.