im cam, a licenced diagnostic tech.
p0171 is a "Bank 1 fuel system lean"
This means that your 1 side of the engine is running lean.
the fuel trims and oxygen sensors will have to be checked.
possible problems that are associated with this code are:
1.intake manifold gasket
if the intake manifold gasket is leaking, this will draw in air and cause a lean running bank. the gasket would have to be replaced.
2. oxygen sensor
IF the oxygen sensor is lazy or faulty, this will give a false lean code as the sensor cannot monitor the fuel trims.
3. plugged fuel injectors.
if the injectors are plugging up, a bad spray will cause a lean condition. they will have to be cleaned or replaced.
these are the most common areas of this code.
since it is only one bank that is lean, it is unlikely that it is a mass airflow sensor.
your mechanic will be able to runa diagnostic test and compare both banks and pinpoint why your engine is running lean on 1 side.
If you need any more assistance id be happy to help
let me know what you find!