My molar had a crack that has recently been filled and now had crown prep. I am worried that it needs a root canal.

Your tooth has been through a significant amount of trauma recently. It is not unusual to have sensitivity after a filling and crown prep. There could also be an issue with your temporary crown not fitting well and causing your tooth to be sensitive. I would recommend you go along with your dentists plan of seating the definitive crown and waiting a little while. Most of the time the sensitivity subsides relatively quickly but if not, it is possible that the trauma to your tooth has damaged your nerve and it may need to be removed (root canal.) Even if this is the case, root canals can be easily performed through a small hole in the top of your new crown then filled back in with filling material. 

Hi:My suggestion is to consult with a root canal specialist to evaluate the tooth for root canal prior to permenant corwn because of your symptoms.Best,Dr. Maddahi

The tooth has been worked on twice, once for the filling and once for the crown prep. It is likely sensitive from the dental work. I would recommend you have the bite and fit of the temporary crown checked and make sure it has been cemented with plenty of temporary cement. I would wait until the pain subsides completely before cementing the permanent crown on or place the permanent crown on with temporary cement. That way it can be removed in the event that root canal treatment is necessary.  This prevents having to drill a hole through your nice new crown and possibly compromising its structural integrity if it is a porcelain or ceramic crown.