Lexus TechDoc

DTC P0171 System Too Lean (Bank 1)
DTC P0172 System Too Rich (Bank 1)

for Preparation Click here

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	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 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 (sensor 1) circuit Air fuel ratio sensor (sensor 1) Air fuel ratio sensor heater (sensor 1) PCV valve and hose PCV hose connections EGR valve assembly ECM Wire harness or connector
P0172	With warm engine and stable air fuel ratio feedback, fuel trim considerably in error to rich side (2 trip detection logic).	Fuel injector assembly 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 (sensor 1) circuit Air fuel ratio sensor (sensor 1) Air fuel ratio sensor heater (sensor 1) EGR valve assembly ECM Wire harness or connector

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 then illuminates.
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, a fuel injection volume that deviates from that estimated by the ECM causes 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 value. Deviations from the ECM's estimated fuel injection volume 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 a malfunction threshold, the ECM interprets this 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.

MONITOR STRATEGY

Required Sensors/Components	Air fuel ratio sensor Mass air flow meter sub-assembly Crankshaft position sensor
Frequency of Operation	Continuous

TYPICAL ENABLING CONDITIONS

Fuel system status	Closed-loop
Auxiliary battery voltage	11 V or higher
Either of the following conditions 1 or 2 is set	-
1. Engine speed	Less than 1100 rpm
2. Intake air amount per revolution	0.12 g/rev or more

TYPICAL MALFUNCTION THRESHOLDS

Purge-cut	Executing
Either of the following conditions 1 or 2 is met	-
1. Average between short-term fuel trim and long-term fuel trim	35% or more (varies with engine coolant temperature)
2. Average between short-term fuel trim and long-term fuel trim	-35% or less (varies with engine coolant temperature)

CONFIRMATION DRIVING PATTERN

Connect the intelligent tester to the DLC3.
Turn the power switch on (IG) and turn the tester on.
Clear the DTCs (even if no DTCs are stored, perform the clear DTC operation).
Turn the power switch off and wait for 30 seconds.
Turn the power switch on (IG) and turn the tester on [A].
Put the engine in inspection mode (maintenance mode) (Click here).
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 between 75 and 120 km/h (47 and 75 mph) and at an engine speed of between 1400 and 3200 rpm 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.
Read the pending DTC [E].
HINT:
1. If a pending DTC is output, the system is malfunctioning.

WIRING DIAGRAM

Air fuel ratio sensor circuit: Refer to DTC P2195 (Click here).
Mass air flow meter circuit: Refer to DTC P0102 (Click here).

INSPECTION PROCEDURE

HINT:

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.
Sensor 1 refers to the sensor closest to the engine assembly.
Sensor 2 refers to the sensor farthest away from the engine assembly.

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

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

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

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

GO TO DTC CHART (Click here)

A

2.CHECK PCV HOSE CONNECTIONS

Inspect the PCV hose connections (Click here).
OK:
PCV hose is connected correctly and is not damaged.

REPAIR OR REPLACE PCV HOSE

OK

3.CHECK INTAKE SYSTEM

Check the intake system for vacuum leaks (Click here).
OK:
No leaks in intake system.

REPAIR OR REPLACE INTAKE SYSTEM

OK

4.PERFORM ACTIVE TEST USING INTELLIGENT TESTER (CONTROL THE INJECTION VOLUME)

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Put the engine in inspection mode (maintenance mode) (Click here).

Start the engine.

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.

Perform the Control the Injection Volume operation with the engine idling.

Monitor the output voltages of the air fuel ratio and heated oxygen sensors (AFS Voltage B1S1 and O2S B1S2) displayed on the tester.

HINT:

Change the fuel injection volume within the range of -12.0% to +12.0%. The injection volume can be changed in fine gradations.
The air fuel ratio sensor has an output delay of a few seconds and the heated oxygen sensor has a maximum output delay of approximately 20 seconds.
If the sensor output voltage does not change (almost no reaction) while performing the Active Test, the sensor may be malfunctioning.

Standard
Tester Display (Sensor)	Injection Volume	Status	Voltage
AFS Voltage B1S1 (Air fuel ratio)	+12%	Rich	Below 3.1 V
AFS Voltage B1S1 (Air fuel ratio)	-12%	Lean	Higher than 3.4 V
O2S B1S2 (Heated oxygen)	+12%	Rich	Higher than 0.55 V
O2S B1S2 (Heated oxygen)	-12%	Lean	Below 0.4 V

Result
Status of AFS Voltage B1S1	Status of O2S B1S2	Air Fuel Ratio Condition and Air Fuel Ratio Sensor Condition	Misfire	Suspected Trouble Area	Proceed to
Lean/Rich	Lean/Rich	Normal	-	-	A
Lean	Lean	Actual air fuel ratio lean	May occur	PCV valve and hose PCV hose connections Fuel injector assembly blockage Gas leaks from exhaust system Intake system Fuel pressure Mass air flow meter sub-assembly Engine coolant temperature sensor EGR valve assembly	A
Rich	Rich	Actual air fuel ratio rich	-	Fuel injector assembly leakage or blockage Gas leaks from exhaust system Ignition system Fuel pressure Mass air flow meter sub-assembly Engine coolant temperature sensor	A
Lean	Lean/Rich	Air fuel ratio sensor malfunction	-	Air fuel ratio sensor EGR valve assembly	B
Rich	Lean/Rich	Air fuel ratio sensor malfunction	-	Air fuel ratio sensor	B

Lean: During the Control the Injection Volume Active Test, the air fuel ratio sensor output voltage (AFS Voltage) is consistently higher than 3.4 V, and the heated oxygen sensor output voltage (O2S) is consistently below 0.4 V.
Rich: During the Control the Injection Volume Active Test, the AFS Voltage is consistently below 3.1 V, and the O2S is consistently higher than 0.55 V.
Lean/Rich: During the Control the Injection Volume Active Test, the output voltage of the heated oxygen sensor alternates correctly.

HINT:

Refer to "Data List / Active Test" [AFS Voltage B1S1 and O2S B1S2] (Click here).

Go to step 11

A

5.READ VALUE USING INTELLIGENT TESTER (COOLANT TEMP)

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Enter the following menus: Powertrain / Engine and ECT / Data List / Coolant Temp.

Read the Data List twice, when the engine is both cold and warmed up.

Standard
Tester Display
Condition
Specified Condition
Coolant Temp
Cold engine
Same as ambient air temperature
Warm engine
Between 75 and 100°C (167 and 212°F)

Standard
Tester Display	Condition	Specified Condition
Coolant Temp	Cold engine	Same as ambient air temperature
Warm engine	Between 75 and 100°C (167 and 212°F)

REPLACE ENGINE COOLANT TEMPERATURE SENSOR (Click here)

OK

6.READ VALUE USING INTELLIGENT TESTER (MAF)

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Put the engine in inspection mode (maintenance mode) (Click here).

Start the engine.

Enter the following menus: Powertrain / Engine and ECT / Data List / All Data / MAF and Coolant Temp.

Allow the engine to idle until Coolant Temp reaches 75°C (167°F) or higher.

Read MAF with the engine speed at 2500 rpm.

Standard
Tester Display
Condition
Specified Condition
MAF
Shift lever position: P
Engine Speed: 2500 rpm
Between 4.5 gm/sec. and 8.5 gm/sec.

Standard
Tester Display	Condition	Specified Condition
MAF	Shift lever position: P Engine Speed: 2500 rpm	Between 4.5 gm/sec. and 8.5 gm/sec.

Go to step 18

OK

7.CHECK FUEL PRESSURE

Check the fuel pressure (Click here).

REPAIR OR REPLACE FUEL SYSTEM

OK

8.CHECK FOR EXHAUST GAS LEAK

Check for exhaust gas leaks.
OK:
No gas leaks.

REPAIR OR REPLACE EXHAUST SYSTEM

OK

9.CHECK FOR SPARKS AND IGNITION

Perform a spark test.

HINT:

Refer to the ignition system inspection procedure (Click here).
If the spark plugs or ignition system malfunctions, engine misfire may occur. The misfire count can be read using the intelligent tester. Enter the following menus: Powertrain / Engine and ECT / Data List / Cylinder #1 Misfire Count (to Cylinder #4 Misfire Count).

REPAIR OR REPLACE IGNITION SYSTEM

OK

10.INSPECT FUEL INJECTOR ASSEMBLY (INJECTION AND VOLUME)

Check the injection and volume.

HINT:

Refer to the fuel injector assembly inspection procedure (Click here).
If the injectors malfunction, engine misfire may occur. The misfire count can be read using the intelligent tester. Enter the following menus: Powertrain / Engine and ECT / Data List / Cylinder #1 Misfire Count (to Cylinder #4 Misfire Count).

Go to step 15

REPLACE FUEL INJECTOR ASSEMBLY (Click here)

11.INSPECT AIR FUEL RATIO SENSOR (HEATER RESISTANCE)

Inspect the air fuel ratio sensor (Click here).

REPLACE AIR FUEL RATIO SENSOR (Click here)

OK

12.CHECK TERMINAL VOLTAGE (+B OF AIR FUEL RATIO SENSOR)

Disconnect the air fuel ratio sensor connector.

Turn the power switch on (IG).

Measure the voltage according to the value(s) in the table below.
Standard Voltage:
Tester Connection
Switch Condition
Specified Condition
D23-2 (+B) - Body ground
Power switch on (IG)
11 to 14 V
Text in Illustration
*a
Front view of wire harness connector
(to Air Fuel Ratio Sensor (Sensor 1))

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

OK

13.CHECK HARNESS AND CONNECTOR (AIR FUEL RATIO SENSOR - 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:

Tester Connection	Condition	Specified Condition
D23-1 (HA1A) - D27-18 (HA1A)	Always	Below 1 Ω
D23-3 (A1A+) - D27-103 (A1A+)	Always	Below 1 Ω
D23-4 (A1A-) - D27-126 (A1A-)	Always	Below 1 Ω
D23-1 (HA1A) or D27-18 (HA1A) - Body ground	Always	10 kΩ or higher
D23-3 (A1A+) or D27-103 (A1A+) - Body ground	Always	10 kΩ or higher
D23-4 (A1A-) or D27-126 (A1A-) - Body ground	Always	10 kΩ or higher

REPAIR OR REPLACE HARNESS OR CONNECTOR

OK

14.REPLACE AIR FUEL RATIO SENSOR

Replace the air fuel ratio sensor (Click here).

NEXT

15.PERFORM CONFIRMATION DRIVING PATTERN

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Clear the DTCs (Click here).

Drive the vehicle in accordance with the driving pattern described in the Confirmation Driving Pattern.

Enter the following menus: Powertrain / Engine and ECT / DTC / Pending.

Read the pending DTCs.

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

END

A

16.PERFORM ACTIVE TEST USING INTELLIGENT TESTER (CONTROL THE EGR STEP POSITION)

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Put the engine in inspection mode (maintenance mode) (Click here).

Start the engine and warm it up until the engine coolant temperature reaches 75°C (167°F) or higher.
HINT:
The A/C switch and all accessory switches should be off.

Enter the following menus: Powertrain / Engine and ECT / Active Test / Control the EGR Step Position / Data List / All Data / Throttle Idle Position and MAP.

Confirm that the Throttle Idle Position is ON and check the engine idling condition and MAP values in the Data List while performing the Active Test.

HINT:

Do not leave the EGR valve open for 10 seconds or more during the Active Test.
Be sure to return the EGR valve to step 0 when the Active Test is completed.
Do not open the EGR valve 30 steps or more during the Active Test.

OK:: MAP and idling condition change in response to EGR step position when Throttle Idle Position is ON in Data List.

Standard:

-	EGR Step Position (Active Test)
-	0 Steps	0 to 30 Steps
Idling condition	Steady idling	Idling changes from steady to rough idling or stalls
MAP (Data List)	MAP value is 20 to 40 kPa (150 to 300 mmHg) (EGR valve is fully closed)	MAP value is at least +10 kPa (75 mmHg) higher than when EGR valve is fully closed

HINT:

During Active Test, if the idling condition does not change in response to EGR step position, then there is probably a malfunction in the EGR valve.

Go to step 18

Go to step 17

17.INSPECT EGR VALVE ASSEMBLY

Remove the EGR valve assembly (Click here).

Check if the EGR valve is stuck open.
OK:
EGR valve is tightly closed.

REPLACE EGR VALVE ASSEMBLY (Click here)

OK

18.CHECK HARNESS AND CONNECTOR

Check the connection and terminal contact pressure of connectors and wire harnesses between the mass air flow meter sub-assembly and ECM (Click here).
HINT:
Repair any problems.

NEXT

19.CHECK WHETHER DTC OUTPUT RECURS

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Clear the DTCs (Click here).

Drive the vehicle in accordance with the driving pattern described in the Confirmation Driving Pattern.

Enter the following menus: Powertrain / Engine and ECT / DTC.

Read the pending DTCs.
Result
Result
Proceed to
DTC P0171 or P0172 is output
A
DTC is not output
B

END

A

20.CHECK HARNESS AND CONNECTOR (MASS AIR FLOW METER SUB-ASSEMBLY - ECM)

Disconnect the mass air flow meter sub-assembly connector.

Disconnect the ECM connector.

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

Standard Resistance:

Tester Connection
Condition
Specified Condition
D5-5 (VG) - D27-94 (VG)
Always
Below 1 Ω
D5-4 (E2G) - D27-117 (E2G)
Always
Below 1 Ω
D5-5 (VG) or D27-94 (VG) - Body ground
Always
10 kΩ or higher

REPAIR OR REPLACE HARNESS OR CONNECTOR

OK

21.REPLACE MASS AIR FLOW METER SUB-ASSEMBLY

Replace the mass air flow meter sub-assembly (Click here).
HINT:
If the result of the inspection performed in step 6 indicated no problem, proceed to the next step without replacing the mass air flow meter sub-assembly.

NEXT

22.CONFIRM WHETHER MALFUNCTION HAS BEEN SUCCESSFULLY REPAIRED

Connect the intelligent tester to the DLC3.

Turn the power switch on (IG).

Turn the tester on.

Clear the DTC (Click here).

Drive the vehicle in accordance with the driving pattern described in the Confirmation Driving Pattern.

Enter the following menus: Powertrain / Engine and ECT / DTC.

Read the pending DTCs.
Result
Result
Proceed to
DTC is not output
A
DTC P0171 or P0172 is output
B

REPLACE ECM (Click here)

END

Tester Connection	Switch Condition	Specified Condition
D23-2 (+B) - Body ground	Power switch on (IG)	11 to 14 V

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

DTC P0171 System Too Lean (Bank 1)

DTC P0172 System Too Rich (Bank 1)