Toyota Repair: Corolla check engine light

Question
I have a 2005 Corolla with 150,000 miles on it.  The check engine light is on steady, so I put a code reader and it is P0171 - fuel injection system lean.  But that's as far as any book goes with it.  Which component could be malfunctioning?

Answer
The lean condition may be caused by a vacuum leak at any of the vacuum switching valves or a vacuum hose connected to the air filter or other components visually inspect the hoses, after that the most common failure is bad mass airflow sensor, it's located on top of the intake hose assembly between the throttle body and close to the air filter, it has an electrical connector with a few wires on and two phillips screws, replace this sensor.