How does a 3 year old root canal become infected?

Leakage: If you have a defective restoration (filling) or crown, bacteria can seep in and create a new bacterial infection. If this happens, the root canal will need to be retreated. Another possibility is that the root is fractured and in that case, the tooth would need to be extracted and replaced with either an implant or bridge.

Bacteria: Somehow, bacteria have re-infected the tip of the root, either through leakage from a defective restoration, a fracture, or dormant bacteria from the first root canal. Root canal therapy is a repair of a damaged tooth, with no guarantees. Have your dentist evaluate your situation and advise on the best course of action.