I'm pretty sure that's the same code I pulled on the wife's 01 dakota. She has the 4.7L which has O2 sensors before and after each cat on the Y-pipe, but I'd imagine that the 3.9 is set up the same way. Anywho, it was the sensor after the cat on the passenger side. You can only get the sensor from the dealer and it runs $80+ dollars. Make sure to pick up an O2 socket for the wrench and a thread chase as well as a bottle of blaster.