Upper molar teeth where i had root canals years ago giving excruciating pain. i thought root canals made them non feel-able?

Root canal: Like any other surgical procedure, root canals are not infallible. Sometimes revision is required. Return to the Dentist who did the treatment, or to a root canal specialist (Endodontist, with specialized diagnostic/treatment equipment/training), for reevaluation. To be effective tooth has to be sealed at both ends, both at the root end and usually a crown at the chewing end.