Second root canal on the same molar and there is still throbbing, pain, and extreme sensitivity to the tooth. Why?

Hi:Sorry to hear about your pain.  There is a possibility of cracked tooth.  It is difficult to see the crack on X-ray, sometime exploratory surgery is necessary to find out if the tooth is cracked.  Best,Dr. Maddahi

I am so sorry to hear what you are going through.  You may have an underlying bite problem that is undiagnosed.  When biting forces are not distributed properly in the entire mouth then teeth with the most pressure can act up.  Even though a root canal was done there is a ligament around the tooth that holds the tooth in the socket.  This ligament is called the periodontal ligament and has very fine receptors.  With pressure these ligaments can stay inflamed and cause pain.  Your occlusion needs to be addressed.  Occlusion is the dental term for how the teeth come together.  There are dentists with advanced training in Occlusion.  One such program is by the Pankey Institute.  You can find a Pankey trained dentist in your area.  Hope this helps.  Good Luck!