P0171 - System Too Lean (Bank 1)
P0174 - System Too Lean (Bank 2)
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.
Exhaust System.
Exhaust leaks before or near the HO2Ss (exhaust manifold gasket, mating gaskets).
EGR system.
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).