What does code P0171 mean on a 1999 Mercury Mystique?

Answer 1

P0171 is "system lean-bank 1", and means your car's computer is having a hard time keeping the mixture right. Right now, it is adding all the fuel it can and still not making it to the correct ratio. This can be caused by a number of factors, but the most common are vacuum leaks in the intake tract, or a failed Mass Airflow Sensor. A decent diagnositic tech should be able to nail the exact cause down pretty quickly.