Lean air-fuel mixture
A lean code means the fuel trims have added fuel for too long because the mixture looked short of fuel or long on air.
P0171 is bank 1, P0174 is bank 2. Vacuum leaks, a dirty mass-airflow sensor, low fuel pressure, or an exhaust leak ahead of the oxygen sensor are typical directions.
The code page lists those checks. Do not treat the code as an order to replace the injectors first.
A trouble code marks a condition to test. It does not by itself prove which part to replace.