P0171 is a DTC (trouble code) triggered by the O2 sensor (AKA oxygen or lambda sensor) located in the exhaust to report that the sensor has detected that the air/fuel mixture injected in the cylinders contains too much air.
If there are no other symptoms and the code does not come back after resetting (codes are reset after scanning), then usually there is nothing to worry about. In some case temporary faults may result in a lean code.
If the code appears again there are different things to check.
A lean code means not enough fuel in mixture. That can be caused by a defective injector, by fuel circulation problems, by electronic injection mixing fuel and air in the wrong way or by an open vacuum hose aspiring extra air into the engine.
The technical description of P0171 From service documentation says:
Lean shift in fuel system operation - P0171 (Bank 1)
The Adaptive Fuel Strategy Fuel System continuously monitors fuel delivery hardware. The code is set when the adaptive fuel tables reach a rich •Low fuel pressure or running out of fuel.
•Contaminated fuel injectors
•Low fuel pressure or running out of fuel (fuel pump, filter, fuel supply line restrictions)
•Vapor recovery system (VMV)
Check the injectors and do an injector cleanup.
Test fuel pressure. The test is done using a fuel gauge like this: Lisle 20300
. Your car's fuel pressure must be between 35/45 PSI.
Check air intake condition, make sure the assembly is all in one piece and that there are no air leaks. Replace filter and clean the MAF sensor in the intake. A defective sensor or a sensor with dirty screen may result in incorrect fuel mixture.
Check the vacuum system. A leak in one of the vacuum hose will trigger a Lean code. Big vacuum leaks can be spotted at a glance. Small leaks can be detected spraying carb cleaner in the vacuum hoses. If there is a leak the engine will rev up a bit when the leak is temporarily obstructed by carburetor cleaner. The gauge can also be used to check if pressure in vacuum hose go up.
Check the Vapor recovery system and positive crankcase ventilation (PCV) system. Check vacuum connected to VMV system and check all the parts comprised in PCV system. Check also that EGR valve is not stuck closed and check the idle air control (IAC) valve.
Below a picture of the parts comprised in PCV system:
Here a picture showing position of IAC valve, see the arrows. Click on image to enlarge: