P0171 - System Too Lean (Bank 1) : The adaptive fuel strategy continuously monitors the fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit.
Causes:
Fuel System.
Fuel pressure regulator (leaking, faulty).
Fuel filter plugged, dirty.
Fuel pump (weak, check valve leaking).
Leaking/contaminated fuel injectors.
Low fuel pressure or running out of fuel.
EVAP canister purge valve leaking (when the canister is clean).
Fuel supply line restricted.
Fuel rail pressure sensor (incorrect reading).
Air Induction System.
Air leaks after the MAF.
Vacuum leaks.
PCV system (leak or valve stuck open)
Improperly seated engine oil dipstick.
Air induction turbulence due to incorrect air filter.
Exhaust System.
Exhaust leaks before or near the HO2Ss (exhaust manifold gasket, mating gaskets)
systEGR em.
Vacuum hose disconnected on EGR System Module (ESM) applications
EGR valve tube/gasket leak
EVR solenoid vacuum leak
Secondary Air Injection.
Damaged/malfunctioning secondary air injection system (mechanically stuck valve).
Air Measurement System.
MAF sensor (contaminated, damaged, or faulty)
It hard to narrow it down without the freeze frame data from the scanner, but these are very easy checks