Where is the shift sensor located and how do I repair it

Do you mean one of the shift solenoids? If yes, the transmission has to come out of the car to replace it. If the smog shop was just noting the code store in the system, it would still be a good idea to get the code diagnosed before buying parts for it. Call a local shop in your area for a better idea of cost. http://repairpal.com/pontiac-within-10-miles-of-palmdale-california-93550