P0171 Code: Meaning, Causes, and Symptoms | OBD2 Codes

P0171 – System Too Lean Bank 1

P0171 Possible Causes

Intake air leaks

Faulty front heated oxygen sensor

Ignition misfiring

Faulty fuel injectors

Exhaust gas leaks

Incorrect fuel pressure

Lack of fuel

Faulty Mass Air Flow (MAF) sensor

Incorrect Positive Crankcase Ventilation (PCV) hose connection

P0171 Possible Symptoms

Engine Light ON (or Service Engine Soon Warning Light)

Excessive Fuel Consumption


With the Air/Fuel Mixture Ratio Self-Learning Control, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The Engine Control Module (ECM) calculates the necessary compensation to correct the offset between the actual and the theoretical ratios. In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the ECM judges the condition as the fuel injection system malfunction and light up the Malfunction Indicator Light (MIL) (2 trip detection logic).