Per TSB, some of the causes of P0171 and P0174, 
P0171 - System Too Lean (Bank 1) The Adaptive Fuel Strategy continuously monitors fuel delivery hardware. The code is set when the adaptive fuel tables reach a rich calibrated limit. 
Fuel System: 
· Contaminated fuel injectors 
· Low fuel pressure or running out of fuel (fuel pump, filter, fuel supply line restrictions) 
· Vapor recovery system (VMV) 
Induction System: 
· MAF contamination 
· Air leaks between the MAF and throttle body 
· Vacuum leaks 
· PCV system concern 
· Improperly seated engine oil dipstick 
EGR System: 
· Leaking gasket 
· Stuck EGR valve 
· Leaking diaphragm or EVR 
Base Engine: 
· Exhaust leaks before or near the HO2S 
· Secondary air concern 
Powertrain Control System: 
· PCM concern 
P0174 - System Too Lean (Bank 2) Same as DTC P0171, but Bank 2. See Possible Causes for DTC P0171