the code p0171 fuel trim too lean relates to the engine ecu detecting too much air being burnt in the engine cylinders thus compensating by increasing the fuel injected into the cylinders to give a richer mixture which throws up the trouble code.
A difficult problem to solve if your engine has no running symptoms as many things could cause this code such as air leaks on manifold and rocker cover, faulty injectors, fuel pressure regulator or pump, air flow meter, lambda sensors and engine ecu to name a few.
in my case here the fault was rectified by replacing a faulty charcoal canister purge (evaporative) solenoid.