Does my dentist need to do a new impression after root canal?

Hi:If the margin of the tooth was not altered during the root canal treatment, then the same impression can be used to make the final restoration.Best,Dr. Maddahi

Not necessarily. It all depends on how much the tooth's shape changed  after the root canal therapy. if the endodontist only made a small occlusal opening, you might not need a new impression. Your dentist should be able to check the fit before cementing the crown.