Jetta 02 1.8T

I have a 2002 Jetter 1.8T. The check engine light keeps coming on. The code is P0171 running lean. One dealer had the car for three months replaced all kinds of parts. Carried to another VW dealer and they have replaced cat coverter, pulled the head and checked it (it was fine), have replaced several sensors and usually before we get home from the dealership the light is back on and with the same code. This time we drove the car for 166 miles before the light came back on, but it is back on again. I am getting really tired of it. It is getting very expensive. The car runs perfect, just the check engine light. Can you help me?