Failing root canal can be diagnosed by either subjective or objective clinical symptoms, and clinical investigations and radiographs. So usually we see patients coming with pain and treated root canal which was probably don earlier. So we do check the history of root canal and we do certain clinical test to see if there is a pocket depth that is increasing or percussion tests that has to be checked and x-rays. The x-rays gives an idea of how the root canal was done and mainly what we see in the x-rays is the filling, that is the obturation is the only thing that you can see that if it is done well or it is not done well. inspite of that we see that root canals are not done and there are failures that happen that are due to other reasons that happen during cleaning, shaping, irrigation process of the tooth, inspite of all that it could be because of the crown that is placed after the root canal is done because lot of studies that are done have shown that the endodontic therapy is very much dependent on how the prosthetic process has gone after that. If there is no good integrity between the crown and the root canal tooth, then the failure of the root canal is much higher. So that is how the root canal has failed. So that is how you take it forward to say that the prognosis for that tooth is good and a re- RCT has been initiated and a better crown has to be put or along with a re- RCT and a crown if there is some apicectomy that has to be done to save the tooth can be planned.