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): Description

WARNING: This page is about a different car, the 2007 Lexus SC 430. However, it is still accessible from the selected car via links, so may be relevant.

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

The short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at the stoichiometric air-fuel ratio. The signal from the heated oxygen sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the stoichiometric air-fuel ratio. This variance triggers a reduction in the fuel volume if the air-fuel ratio is RICH, and an increase in the fuel volume if it is LEAN.

The short-term fuel trim varies from the central value due to individual engine differences, wear over time and changes in the operating environment. The long-term fuel trim, which controls overall fuel compensation, steadies long-term deviations of the short-term fuel trim from the central value.

If both the short-term fuel trim and the long-term fuel trim are LEAN or RICH beyond a certain value, the ECM determines this as a malfunction, illuminates the MIL and sets a DTC.

DTC DETECTION CONDITION CHART

DTC No.	DTC Detection Condition	Trouble Area
P0171 P0174	When air-fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on LEAN side (2 trip detection logic)	Air induction system Injector blockage MAF meter ECT sensor Fuel pressure Gas leakage in exhaust system Heated oxygen sensor (sensor 1) Heated oxygen sensor (sensor 1) heater EFI relay PCV valve and hose ECM
P0172 P0175	When air-fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on RICH side (2 trip detection logic)	Injector leak or blockage MAF meter ECT sensor Ignition system Fuel pressure Gas leakage in exhaust system Heated oxygen sensor (sensor 1) Heated oxygen sensor (sensor 1) heater EFI relay ECM

HINT:

When DTC P0171 or P0174 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 or P0175 is recorded, 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 recorded. The MIL is then illuminated.
If the total of the short-term fuel trim value and long-term fuel trim value is within the malfunction threshold (and ECT is more than 75°C (167°F)), the system is functioning normally.