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 the 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 (sensor 1) 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 change 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 No.	DTC Detection Condition	Trouble Area
P0171	With a warm engine and stable air fuel ratio feedback, fuel trim is too lean (2 trip detection logic)	Intake system Fuel pressure Injector blockage Mass air flow meter Engine coolant temperature sensor Gas leakage from exhaust system Open or short in heated oxygen sensor (sensor 1) circuit Air fuel ratio sensor (sensor 1) Air fuel ratio sensor heater (sensor 1) Air fuel ratio sensor heater circuits PCV hose connections PCV valve and hose ECM
P0172	With a warm engine and stable air fuel ratio feedback, fuel trim is too rich (2 trip detection logic)	Injector leakage or blockage Fuel pressure Mass air flow meter Engine coolant temperature sensor Ignition system Gas leakage from exhaust system Open or short in heated oxygen sensor (sensor 1) circuit Air fuel ratio sensor (sensor 1) Air fuel ratio sensor heater (sensor 1) Air fuel ratio sensor heater circuits Ignition system ECM

DTC No.

DTC Detection Condition

Trouble Area

P0171

With a warm engine and stable air fuel ratio feedback, fuel trim is too lean

(2 trip detection logic)

Intake system
Fuel pressure
Injector blockage
Mass air flow meter
Engine coolant temperature sensor
Gas leakage from exhaust system
Open or short in heated oxygen sensor (sensor 1) circuit
Air fuel ratio sensor (sensor 1)
Air fuel ratio sensor heater (sensor 1)
Air fuel ratio sensor heater circuits
PCV hose connections
PCV valve and hose
ECM

P0172

With a warm engine and stable air fuel ratio feedback, fuel trim is too rich

(2 trip detection logic)

Injector leakage or blockage
Fuel pressure
Mass air flow meter
Engine coolant temperature sensor
Ignition system
Gas leakage from exhaust system
Open or short in heated oxygen sensor (sensor 1) circuit
Air fuel ratio sensor (sensor 1)
Air fuel ratio sensor heater (sensor 1)
Air fuel ratio sensor heater circuits
Ignition system
ECM

Tech Tips

When DTC P0171 is set, the actual air fuel ratio is too lean. When DTC P0172 is set, the actual air fuel ratio is too rich.
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 20% of the central value (and the engine coolant temperature is more 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's estimated fuel injection volumes also affect the average fuel trim learning value, which is a combination of the average short-term fuel trim (fuel feedback compensation value) and the average long-term fuel trim (learning value of the air fuel ratio). If the average fuel trim learning value exceeds the malfunction threshold, the ECM interprets this as a fault in the fuel system and sets a DTC.

Example:

The average fuel trim learning value is +35% or more or -35% or less, the ECM interprets this as a fuel system malfunction.

WIRING DIAGRAM

Refer to DTC P2195 Click here.

INSPECTION PROCEDURE

Note

Inspect the fuses for circuits related to this system before performing the following inspection procedure.

Tech Tips

Read freeze frame data using the intelligent tester. The ECM records vehicle and driving condition information as freeze frame data the moment a DTC is stored. When troubleshooting, freeze frame data can help determine if the vehicle was moving or stationary, if the engine was warmed up or not, if the air fuel ratio was lean or rich, and other data from the time the malfunction occurred.
A low air fuel ratio sensor voltage could be caused by a rich air fuel mixture. Check for conditions that would cause the engine to run rich.
A high air fuel ratio sensor voltage could be caused by a lean air fuel mixture. Check for conditions that would cause the engine to run lean.

PROCEDURE

CHECK ANY OTHER DTCS OUTPUT (IN ADDITION TO DTC P0171 OR P0172)

Connect the intelligent tester to the DLC3.
Turn the ignition switch to ON.
Turn the tester on.
Enter the following menus: Powertrain / Engine and ECT / DTC.

Read the DTCs.

Result
Result	Proceed to
DTC P0171 or P0172 is output	A
DTC P0171 or P0172 and other DTCs are output	B

Tech Tips

If any DTCs other than P0171 or P0172 are output, troubleshoot those DTCs first.

GO TO DTC CHART Click here

PERFORM ACTIVE TEST USING INTELLIGENT TESTER (AIR FUEL RATIO CONTROL)

Connect the intelligent tester to the DLC3.
Start the engine.
Turn the tester on.
Warm up the engine at an engine speed of 2500 rpm for approximately 90 seconds.
Enter the following menus: Powertrain / Engine and ECT / Active Test / Control the Injection Volume for A/F Sensor.
Perform the "Control the Injection Volume for A/F Sensor" operation with the engine in an idling condition (press the RIGHT or LEFT button to change the fuel injection volume).

Monitor the output voltages of the air fuel ratio sensor (sensor 1) and heated oxygen sensor (sensor 2) (AFS Voltage B1 S1 and O2S B1 S2) displayed on the tester.

Result
The air fuel ratio sensor (sensor 1) reacts in accordance with increases and decreases in the fuel injection volume:
+25% = Rich output
More than 3.4 V
-12.5% = Lean output
Less than 3.1 V

Note

The air fuel ratio sensor (sensor 1) has an output delay of a few seconds and the heated oxygen sensor (sensor 2) has a maximum output delay of approximately 20 seconds.

Case	A/F Sensor (Sensor 1) Output Voltage	HO2 Sensor (Sensor 2) Output Voltage	Main Suspected Trouble Area
1			-
2			A/F sensor A/F sensor heater A/F sensor circuit
3			Injector Fuel pressure Gas leak from exhaust system (Air-fuel ratio extremely rich or lean)

Following the "Control the Injection Volume for A/F Sensor" procedure enables technicians to check and graph the voltage outputs of both the air fuel ratio sensor (sensor 1) and heated oxygen sensor (sensor 2).
To display the graph, enter the following menus: Powertrain / Engine and ECT / Active Test / Control the Injection Volume for A/F Sensor / AFS Voltage B1 S1 and O2S B1 S2.

Result
Result	Proceed to
Case 1	A
Case 2	B
Case 3	C

INSPECT AIR FUEL RATIO SENSOR (HEATER RESISTANCE) Click here

INSPECT FOR EXHAUST GAS LEAK Click here

CONFIRM IF VEHICLE HAS RUN OUT OF FUEL IN PAST
1. Has the vehicle run out of fuel in the past?
NO
INSPECT FOR EXHAUST GAS LEAK Click here

YES
PERFORM CONFIRMATION DRIVING PATTERN
Tech Tips

This confirmation driving pattern is used in the "PERFORM CONFIRMATION DRIVING PATTERN" procedure of the following diagnostic troubleshooting procedure.
- (1) Connect the intelligent tester to the DLC3 (Procedure "A").
- (2) Turn the ignition switch to ON (Procedure "B").
- (3) Turn the tester on (Procedure "C").
- (4) Clear the DTCs Click here (Procedure "D").
- (5) Switch the ECM from normal mode to check mode Click here (Procedure "E").
- (6) Start the engine (Procedure "F").
- (7) Allow the engine to idle until the engine coolant temperature reaches 40°C (104°F) (Procedure "G").
- (8) Drive the vehicle at a vehicle speed of more than 40 km/h (25 mph) for 35 seconds or more (Procedure "H").
- (9) Allow the engine to idle for 40 seconds or more (Procedure "I").
- (10) Repeat procedures "G" and "I" described above at least 3 times (Procedure "J").
- (11) Allow the engine to idle for 40 seconds or more (Procedure "K").
Tech Tips

The MIL is illuminated during procedure "K" if a malfunction still exists.

Note

If the conditions in this test are not strictly followed, malfunctions may not be detected.

NEXT

CHECK WHETHER DTC OUTPUT RECURS (DTC P0171 OR P0172)

Connect the intelligent tester to the DLC3.
Turn the ignition switch to ON.
Turn the tester on.
Enter the following menus: Powertrain / Engine and ECT / DTC.

Read the DTCs.

Result
Result	Proceed to
DTC P0171 or P0172 is output	A
DTC is not output	B

SYSTEM IS OK

INSPECT FOR EXHAUST GAS LEAK
1. Check for exhaust gas leakage.
  
  OK
  
  No gas leakage.
NG
REPAIR OR REPLACE EXHAUST SYSTEM

OK
CHECK FUEL PRESSURE
1. Check the fuel pressure Click here.
NG
CHECK FUEL LINE Click here

OK
INSPECT INJECTOR ASSEMBLY (INJECTION VOLUME)
1. Check the injection volume Click here.
NG
REPLACE FUEL INJECTOR ASSEMBLY Click here

OK
READ VALUE USING INTELLIGENT TESTER (MASS AIR FLOW METER)
1. Inspect the mass air flow meter Click here.
NG
REPLACE MASS AIR FLOW METER Click here

OK

OK
No gas leakage.

READ VALUE USING INTELLIGENT TESTER (COOLANT TEMP)

Connect the intelligent tester to the DLC3.
Turn the ignition switch to ON and turn the tester on.
Enter the following menus: Powertrain / Engine and ECT / Data List / Primary / Coolant Temp.

Read the Coolant Temp twice, when the engine is both cold and warmed up.

Standard
With cold engine: Same as ambient air temperature.
With warm engine: Between 75°C and 100°C (167°F and 212°F).

REPLACE ENGINE COOLANT TEMPERATURE SENSOR Click here

CHECK PCV HOSE CONNECTIONS
1. Check the PCV hose connections Click here.
  
  OK
  
  PCV hose is connected correctly and is not damaged.
NG
REPAIR OR REPLACE PCV HOSE

OK
CHECK INTAKE SYSTEM
1. Check the intake system for vacuum leakage Click here.
  
  OK
  
  No leakage from intake system.
NG
REPAIR OR REPLACE INTAKE SYSTEM

OK
CHECK IGNITION SYSTEM
1. Inspect ignition system Click here.
NG
REPAIR OR REPLACE IGNITION SYSTEM

OK

OK
PCV hose is connected correctly and is not damaged.

OK
No leakage from intake system.

INSPECT AIR FUEL RATIO SENSOR (HEATER RESISTANCE)

Text in Illustration

*1
Component without harness connected

(Air Fuel Ratio Sensor)

Disconnect the air fuel ratio sensor connector.

Text in Illustration
*1	Component without harness connected (Air Fuel Ratio Sensor)

Measure the resistance according to the value(s) in the table below.

Standard Resistance

Tester Connection	Condition	Specified Condition
1 (HA1A) - 2 (+B)	20°C (68°F)	1.8 to 3.4 Ω
1 (HA1A) - 4 (A1A-)	Always	10 kΩ or higher

Reconnect the air fuel ratio sensor.

REPLACE AIR FUEL RATIO SENSOR Click here

CHECK AIR FUEL RATIO SENSOR (POWER SOURCE)

Text in Illustration

*1
Front view of wire harness connector

(to Air Fuel Ratio Sensor)

Disconnect the air fuel ratio sensor connector.
Turn the ignition switch to ON.

Measure the voltage according to the value(s) in the table below.

Standard Voltage

Tester Connection	Switch Condition	Specified Condition
B18-2 (+B) - Body ground	Ignition switch ON	11 to 14 V

Reconnect the air fuel ratio sensor connector.

Click here

CHECK HARNESS AND CONNECTOR (AIR FUEL RATIO SENSOR - ECM)

Text in Illustration
*1	Front view of wire harness connector (to Air Fuel Ratio Sensor)
*2	Front view of wire harness connector (to ECM)

Disconnect the air fuel ratio sensor connector.

Disconnect the ECM connector.

Measure the resistance according to the value(s) in the table below.

Standard Resistance (Check for open)

Tester Connection	Condition	Specified Condition
B18-1 (HA1A) - B39-104 (HA1A)	Always	Below 1 Ω
B18-3 (A1A+) - B39-103 (A1A+)	Always	Below 1 Ω
B18-4 (A1A-) - B39-126 (A1A-)	Always	Below 1 Ω

Standard Resistance (Check for short)

Tester Connection	Condition	Specified Condition
B18-1 (HA1A) or B39-104 (HA1A) - Body ground	Always	10 kΩ or higher
B18-3 (A1A+) or B39-103 (A1A+) - Body ground	Always	10 kΩ or higher
B18-4 (A1A-) or B39-126 (A1A-) - Body ground	Always	10 kΩ or higher

Reconnect the ECM connector.
Reconnect the air fuel ratio sensor connector.

REPAIR OR REPLACE HARNESS OR CONNECTOR (AIR FUEL RATIO SENSOR - ECM)

REPLACE AIR FUEL RATIO SENSOR
1. Replace the air fuel ratio sensor Click here.
NEXT
PERFORM CONFIRMATION DRIVING PATTERN
Tech Tips

This confirmation driving pattern is used in the "PERFORM CONFIRMATION DRIVING PATTERN" procedure of the following diagnostic troubleshooting procedure.
- (1) Connect the intelligent tester to the DLC3 (Procedure "A").
- (2) Turn the ignition switch to ON (Procedure "B").
- (3) Turn the tester on (Procedure "C").
- (4) Clear the DTCs Click here (Procedure "D").
- (5) Switch the ECM from normal mode to check mode Click here (Procedure "E").
- (6) Start the engine (Procedure "F").
- (7) Allow the engine to idle until the engine coolant temperature reaches 40°C (104°F) (Procedure "G").
- (8) Drive the vehicle at a vehicle speed of more than 40 km/h (25 mph) for 35 seconds or more (Procedure "H").
- (9) Allow the engine to idle for 40 seconds or more (Procedure "I").
- (10) Repeat procedure "G" and "I" described above at least 3 times (Procedure "J").
- (11) Allow the engine to idle for 40 seconds or more (Procedure "K").
Tech Tips

The MIL is illuminated during procedure "K" if a malfunction still exists.

Note

If the conditions in this test are not strictly followed, malfunctions may not be detected.

NEXT

CHECK WHETHER DTC OUTPUT RECURS (DTC P0171 OR P0172)

Connect the intelligent tester to the DLC3.
Turn the ignition switch to ON.
Turn the tester on.
Enter the following menus: Powertrain / Engine and ECT / DTC.

Read the DTCs.

Result
Result	Proceed to
DTC is not output	A
DTC P0171 or P0172 is output	B

REPLACE ECM Click here

END

CHECK HARNESS AND CONNECTOR (AIR FUEL RATIO SENSOR - INTEGRATION RELAY)

Text in Illustration
*1	Front view of wire harness connector (to Air Fuel Ratio Sensor)
*2	Front view of wire harness connector (to Integration Relay)

Disconnect the air fuel ratio sensor connector.
Remove the integration relay (EFI MAIN relay) from the engine room No. 1 relay block Click here.

Measure the resistance according to the value(s) in the table below.

Standard Resistance (Check for open)

Tester Connection	Condition	Specified Condition
B18-2 (+B) - 1B-4	Always	Below 1 Ω

Standard Resistance (Check for short)

Tester Connection	Condition	Specified Condition
B18-2 (+B) or 1B-4 - Body ground	Always	10 kΩ or higher

Reconnect the air fuel ratio sensor connector.
Reinstall the integration relay.

REPAIR OR REPLACE HARNESS OR CONNECTOR (AIR FUEL RATIO SENSOR - INTEGRATION RELAY)

GO TO ECM POWER SOURCE CIRCUIT Click here

CHECK FUEL LINE
1. Check the fuel lines for leaks or blockage Click here.
  
  OK
  
  No leaks or blockage.
NG
REPAIR OR REPLACE FUEL LINE

OK

REPLACE FUEL PUMP Click here