When does a tooth need a root canal?

Root canal: It is needed when the blood or nerve supply of the tooth is damaged and /or infected through tooth decay or trauma.

Many reasons: it could be many reasons. you can consult a dentist and get x-rays. some reasons why you need root canal: 1- infection 2- broken tooth. 3- exposed tooth nerve. best way to tell is to get an exam and x-rays by dentist to evaluate

Decay/Pain: If you are having acute pain, decay has reached the nerve chamber, or you do not have enough tooth structure for a crown (post +core) these are all indications for root canal treatment

RCT: RCT is needed most often when the pulpal tissue of the tooth is irreversibly damaged. This could be from decay (caries), trauma (such as a blow to the tooth), bruxism, internal fracture of tooth, or exposed nerve from severe abrasions, etc. Sometimes RCT is done as an elective procedure (due to extreme super-eruption for example).