I have been expericing pain (with cold and hot and sometimes chewing) with a molar that already has crown. I saw a dentist last week who said it might be a problem with the tooth under the crown and that I may need a root canal that would be performed by an endodontist. In the meantime, he said I may have a sinus infection and prescribed sme antibiotics. My question is two fold: 1) if it were an sinus issue wouldn't the other teeth be affected as well and not just one? 2) Would a root canal for a crowned tooth be very painful and compicated? :D

It sounds as though you need a root canal. Get it done. There won't be much pain as they remove the root

It's not uncommon for teeth to die after being crowned. The more work done on a tooth the more irritated the nerve becomes. If you have good freezing the root canal should be as comfortable as having a simple filling done. The nerve of the tooth is removed easily.

There should be no need for a new crown to be done. The DDS can drill right through the crown to access the canals to be treated.