System too Lean

I have a 2000 dodge dakota. The check engine light is on and the code it gives me is P0171 - system too lean (left cylinder bank). Does anyone have any idea where I can start to try to narrow this down to the problem??

#1 cause of po 171 is a vacuum leak so check around. that being said that is not the only cause of po171. mike

Thank you I will definitely start there.

I apologize I forgot to mention that there is also a black soot coming from the tailpipe. The truck has an awful smell.

I have the problem narrowed down to the O2 sensor. Does anyone know how to test the sensor in order to determine if it is upstream or downstream?