SFI SYSTEM, Diagnostic DTC:P0171, P0172

DTC Code	DTC Name
P0171	System Too Lean (Bank 1)
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 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.

DTC No.	Detection Item	DTC Detection Condition	Trouble Area	MIL	Memory
P0171	System Too Lean (Bank 1)	With a warm engine and stable air-fuel ratio feedback, the fuel trim is considerably in error to the 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 leak from exhaust system Open or short in air fuel ratio sensor (Sensor 1) circuit Air fuel ratio sensor (Sensor 1) PCV valve and hose PCV hose connections Wire harness or connector ECM	Comes on / -	DTC stored / -
P0172	System Too Rich (Bank 1)	With a warm engine and stable air-fuel ratio feedback, the fuel trim is considerably in error to the 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 leak from exhaust system Open or short in air fuel ratio sensor (Sensor 1) circuit Air fuel ratio sensor (Sensor 1) Wire harness or connector ECM	Comes on / -	DTC stored / -

Tech Tips

When DTC P0171 is stored, the actual air-fuel ratio is on the lean side. When DTC P0172 is stored, 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 stored. 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 higher than 75°C [167°F]), the system is functioning normally.

MONITOR DESCRIPTION

Under closed-loop fuel control, fuel injection volumes that deviate from those estimated by the ECM cause changes in the long-term fuel trim compensation value. The long-term fuel trim is adjusted when there are persistent deviations in the short-term fuel trim values. Deviations from the ECM estimated fuel injection volumes also affect the average fuel trim learned value, which is a combination of the average short-term fuel trim (fuel feedback compensation value) and the average long-term fuel trim (learned value of the air-fuel ratio). If the average fuel trim learned value exceeds the malfunction threshold, the ECM interprets this as a fault in the fuel system and stores a DTC.

Example:

If the average fuel trim learned value is +48% or more, or -38% or less, the ECM interprets this as a fuel system malfunction.

MONITOR STRATEGY

Required Sensors/Components

Air fuel ratio sensor

Mass air flow meter

Crankshaft position sensor

Frequency of Operation

Continuous

TYPICAL ENABLING CONDITIONS

Fuel system status	Closed loop
Battery voltage	11 V or higher
Either of following conditions 1 or 2 met	-
1. Engine speed	Less than 650 rpm
2. Engine load	11.3% or more

TYPICAL MALFUNCTION THRESHOLDS

Purge-cut	Executing
Either of following conditions 1 or 2 met	-
1. Average of short-term fuel trim and long-term fuel trim	48% or more (varies with engine coolant temperature)
2. Average of short-term fuel trim and long-term fuel trim	-38% or less (varies with engine coolant temperature)

CONFIRMATION DRIVING PATTERN

Connect the GTS to the DLC3.
Turn the ignition switch to ON and turn the GTS on.
Clear the DTCs (even if no DTCs are stored, perform the clear DTC operation).
Turn the ignition switch off and wait for at least 30 seconds.
Turn the ignition switch to ON and turn the GTS on [A].
Start the engine and warm it up until the engine coolant temperature is 75°C (167°F) or higher with all the accessories switched off [B].
With the engine warmed up, idle the engine for 2 minutes or more [C].
Drive the vehicle at a speed between 60 and 100 km/h (38 and 63 mph) for 5 minutes or more [D].

CAUTION:

When performing the confirmation driving pattern, obey all speed limits and traffic laws.
Enter the following menus: Powertrain / Engine and ECT / DTC [E].
Read the pending DTCs.

Tech Tips

If a pending DTC is output, the system is malfunctioning.

WIRING DIAGRAM

Refer to DTC P0031.

GTS Display (Sensor)	Injection Volume	Status	Voltage
AFS Voltage B1S1 (Air fuel ratio)	+12.5%	Rich	Below 3.1 V
AFS Voltage B1S1 (Air fuel ratio)	-12.5%	Lean	Higher than 3.4 V
O2S B1S2 (Heated oxygen)	+12.5%	Rich	Higher than 0.55 V
O2S B1S2 (Heated oxygen)	-12.5%	Lean	Below 0.4 V

Tester Display
Engine Speed
MAF
Coolant Temp