DTC P0171: System Too Lean (Bank 1); DTC P0172: System Too Rich (Bank 1); DTC P0174: System Too Lean (Bank 2); DTC P0175: System Too Rich (Bank 2) (03/2012 - 07/2012): Description

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

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 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 trim are lean or rich beyond predetermined values, it is interpreted as a malfunction, and the ECM illuminates the MIL and sets a DTC.

DTC No.	DTC Detection Condition	Trouble Area
P0171 P0174	With warm engine and stable air fuel ratio feedback, fuel trim considerably in error to lean side (2 trip detection logic)	Intake system Fuel injector assembly blockage Mass air flow meter sub-assembly Engine coolant temperature sensor Fuel pressure Gas leaks from exhaust system Open or short in air fuel ratio sensor (bank 1, 2 sensor 1) circuit Air fuel ratio sensor (bank 1, 2 sensor 1) A/F relay PCV valve and hose PCV hose connections Wire harness or connector ECM
P0172 P0175	With warm engine and stable air fuel ratio feedback, fuel trim considerably in error to rich side (2 trip detection logic)	Fuel injector assembly leak or blockage Mass air flow meter sub-assembly Engine coolant temperature sensor Ignition system Fuel pressure Gas leaks from exhaust system Open or short in air fuel ratio sensor (bank 1, 2 sensor 1) circuit Air fuel ratio sensor (bank 1, 2 sensor 1) A/F relay Wire harness or connector ECM

HINT:

When DTC P0171 or P0174 is set, the actual air fuel ratio is on the lean side. When DTC P0172 or P0175 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 or P0174 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.