Tooth start hurting after 18 months from root canal and crown?

Hi:
My advice is to consult with a root canal specialist to evaluate the tooth.  If infected many times the root canal can be redone to save the tooth.
Best,
Dr. Maddahi

There may be several causes for the root canal treated tooth becoming painful. For some scenarios, such as fractured root, extraction would be the best option. But for others, such as an infection (e.g. due to an extra canal that may have been missed during treatment), it maybe possible to re-treat the tooth through the crown.  
Since it's best to keep your natural tooth for as long as possible, you may want to get another evaluation, possibly by an Endodontist (Root canal specialist) who can perform tests/CT scan to determine the cause.

It sounds like the tooth is non restorable at this point. If the root canal treatment and crown were done and the tooth starts to hurt again. The possibilities are a crack in the tooth or an infection. And in both situations the best outcome would be to remove the tooth and replace it.