DTC P0171 System Too Lean (Bank 1); DTC P0172 System Too Rich (Bank 1): Description

The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim consists of both the short-term and long-term fuel trims.

The short-term fuel trim is fuel compensation that is used to constantly maintain the air-fuel ratio at stoichiometric levels. The signal from the Air-fuel Ratio (A/F) sensor indicates whether the air-fuel ratio is rich or lean compared to the stoichiometric ratio. This triggers a reduction in the fuel injection volume if the air-fuel ratio is rich and an increase in the fuel injection volume if it is lean.

Factors such as individual engine differences, wear over time and changes in operating environment cause short-term fuel trim to vary from the central value. The long-term fuel trim, which controls overall fuel compensation, compensates for long-term deviations in the fuel trim from the central value caused by the short-term fuel trim compensation.

If both the short-term and long-term fuel trims are lean or rich beyond predetermined values, it is interpreted as a malfunction, and the ECM illuminates the MIL and sets a DTC.

DTC DETECTION CONDITIONS AND TROUBLE AREAS REFERENCE

DTC No.	DTC Detection Conditions	Trouble Areas
P0171	With warm engine and stable air-fuel ratio feedback, fuel trim considerably in error to lean side (2 trip detection logic)	Air induction system Injector blockage Mass Air Flow (MAF) meter Engine Coolant Temperature (ECT) sensor Fuel pressure Gas leakage from exhaust system Open or short in A/F sensor (sensor 1) circuit A/F sensor (sensor 1) A/F sensor heater (sensor 1) EFI relay A/F sensor heater and EFI relay circuits PCV valve and hose PCV hose connections ECM
P0172	With warm engine and stable air-fuel ratio feedback, fuel trim considerably in error to rich side (2 trip detection logic)	Injector leakage or blockage MAF meter ECT sensor Ignition system Fuel pressure Gas leakage from exhaust system Open or short in A/F sensor (sensor 1) circuit A/F sensor (sensor 1) A/F sensor neater (sensor 1) EFI relay A/F sensor heater and EFI relay circuits ECM

HINT:

When DTC P0171 is set, the actual air-fuel ratio is on the lean side. When DTC P0172 is set, the actual air-fuel ratio is on the rich side.
If the vehicle runs out of fuel, the air-fuel ratio is lean and DTC P0171 may be set. The MIL is then illuminated.
When the total of the short-term and long-term fuel trim values is within the malfunction threshold (and the engine coolant temperature is more than 75°C [167°F]), the system is functioning normally.