Toyota 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 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 (A/F) 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.	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)	Air induction system Fuel injector blockage Manifold absolute pressure sensor 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 EGR valve assembly ECM
P0172	With warm engine and stable air-fuel ratio feedback, fuel trim considerably in error to rich side (2-trip detection logic)	Fuel injector leak or blockage Manifold absolute pressure sensor 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) EGR valve assembly ECM

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 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.

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 thresholds, the ECM interprets this a fault in the fuel system and sets a DTC.
Example:
The average fuel trim learning value is more than +35% or less than -35%, and the ECM interprets this as a fuel system malfunction.

WIRING DIAGRAM

Refer to DTC P2195 (Click here).

INSPECTION PROCEDURE

HINT:

Intelligent tester only:

Malfunctioning areas can be identified by performing the Control the Injection Volume for A/F sensor function provided in the Active Test. The Control the Injection Volume for A/F sensor function can help to determine whether the Air-Fuel Ratio (A/F) sensor, Heated Oxygen (HO2) sensor and other potential trouble areas are malfunctioning.

The following instructions describe how to conduct the Control the Injection Volume for A/F sensor operation using the intelligent tester.

Connect the intelligent tester to the DLC3.
Start the engine and 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 Active Test 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 A/F and HO2 sensors (AF Voltage B1S1 and O2S B1S2) displayed on the tester.

HINT:

The Control the Injection Volume for A/F sensor operation lowers the fuel injection volume by 12.5% or increases the injection volume by 25%.
Each sensor reacts in accordance with increases and decreases in the fuel injection volume.

Tester Display (Sensor)	Injection Volume	Status	Voltage
AF Voltage B1S1 (A/F)	+25%	Rich	Less than 3.1 V
AF Voltage B1S1 (A/F)	-12.5%	Lean	More than 3.4 V
O2S B1S2 (HO2)	+25%	Rich	More than 0.55 V
O2S B1S2 (HO2)	-12.5%	Lean	Less than 0.4 V

NOTICE:

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.

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			HO2 sensor HO2 sensor heater HO2 sensor circuit
4			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 A/F and HO2 sensors.
To display the graph, enter the following menus: Powertrain / Engine and ECT / Active Test / Control the Injection Volume for A/F Sensor / A/F Control System / AF Voltage B1S1 and O2S B1S2; then press the View button.

HINT:

Read freeze frame data using an intelligent tester. Freeze frame data records the engine condition when malfunctions are detected. 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.

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

Connect the intelligent tester to the DLC3.

Turn the ignition switch to ON.

Turn the intelligent tester on.

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

Read the DTC.

Result
Result	Proceed to
P0171 or P0172	A
P0171 or P0172 and other DTCs	B

GO TO DTC CHART (Click here)

A

2.PERFORM ACTIVE TEST USING INTELLIGENT TESTER (OPERATE EGR VALVE)

Connect the intelligent tester to the DLC3.

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.

Turn the tester on.

Enter the following menus: Powertrain / Engine and ECT / Active Test / Control the EGR Step Position.

Check the MAP in Data List and the idling condition while performing Active Test.
OK:
MAP and idling condition change in response to EGR step position.

Standard
-	EGR Step Position (Active Test)
-	0 Steps	0 to 30 Steps	30 to 40 Steps or Higher
Idling condition	Steady idling	Engine changes from steady to rough idling	Engine changes from rough idling to engine stall
MAP value (Data List)	28 to 48 kPa (EGR valve is fully closed)	MAP value is at least +10 to 15 kPa higher than when EGR valve is fully closed	80 to 110 kPa (After engine stall)

HINT:

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

Result
Result	Proceed to
Outside standard range	A
Within standard range	B

Go to step 4

A

3.CHECK WHETHER DTC OUTPUT RECURS

Connect the intelligent tester to the DLC3.

Turn the ignition switch to ON.

Turn the tester on.

Clear the DTC.

Turn the ignition switch off and wait for 30 seconds.

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.

Switch the ECM from normal mode to check mode.

Drive the vehicle at 60 km/h (37 mph) with the shift lever in B.
CAUTION:
1. Strictly observe speed limits and traffic laws when performing the confirmation drive pattern.

Perform fuel-cut operation for 5 seconds or more, with the accelerator pedal fully released.
HINT:
1. If the deceleration was not completed, perform the confirmation driving pattern again.

Enter the following menus: Powertrain / Engine and ECT / Utility / All Readiness.

Input the DTCs to be confirmed.

Check the DTC judgment result.

Result
Result	Proceed to
No output	A
P0401	B

Intelligent Tester
Tester Display	Description
NORMAL	DTC judgment completed System normal
ABNORMAL	DTC judgment completed System abnormal
INCOMPLETE	DTC judgment not completed You should perform driving pattern after confirming DTC enabling conditions
UNKNOWN	Unable to perform DTC judgment Number of DTCs which do not fulfill DTC preconditions has reached ECU's memory limit

HINT:

If the DTC judgment result is INCOMPLETE or UNKNOWN, drive the vehicle again at 60 km/h (37 mph) with the shift lever in B, perform fuel-cut operation for 5 seconds or more with the accelerator pedal fully released, and check the DTC judgment result.

GO TO DTC CHART (Click here)

A

4.CHECK PCV HOSE CONNECTIONS

OK:: PCV hose is connected correctly and is not damaged.

REPAIR OR REPLACE PCV HOSE

OK

5.CHECK AIR INDUCTION SYSTEM

Check the air induction system for vacuum leak.
OK:
No leak from air induction system.

REPAIR OR REPLACE AIR INDUCTION SYSTEM

OK

6.PERFORM ACTIVE TEST USING INTELLIGENT TESTER (INJECTION VOLUME)

Connect an 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 voltage outputs of the air fuel ratio sensor and heated oxygen sensor (AFS Voltage B1S1 and O2S B1S2) displayed on the tester.

HINT:

The Control the Injection Volume for A/F Sensor operation lowers the fuel injection volume by 12.5% or increases the injection volume by 25%.
Each sensor reacts in accordance with increases and decreases in the fuel injection volume.

Standard:

Tester Display (Sensor)	Injection Volumes	Status	Voltage
AFS Voltage B1S1 (A/F)	+25%	Rich	Less than 3.1 V
AFS Voltage B1S1 (A/F)	-12.5%	Lean	More than 3.4 V
O2S B1S2 (HO2)	+25%	Rich	More than 0.55 V
O2S B1S2 (HO2)	-12.5%	Lean	Less than 0.4 V

Result
Status AFS Voltage B1S1	Status O2S B1S2	A/F Condition and A/F Sensor Condition	Misfire	Suspected Trouble Area	Proceed to
Lean/Rich	Lean/Rich	Normal	-	-	C
Lean	Lean	Actual air fuel ratio lean	May occur	PCV valve and hose PCV hose connections Fuel injector blockage Gas leak from exhaust system Air induction system Fuel pressure Manifold Absolute Pressure Sensor Engine coolant temperature sensor	A
Rich	Rich	Actual air fuel ratio rich	-	Fuel injector leak or blockage Gas leak from exhaust system Ignition system Fuel pressure Manifold Absolute Pressure Sensor Engine coolant temperature sensor	A
Lean	Lean/Rich	Air fuel ratio sensor malfunction	-	Air fuel ratio sensor	B
Rich	Lean/Rich	Air fuel ratio sensor malfunction	-	Air fuel ratio sensor	B

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

Go to step 13

Go to step 16

A

7.READ VALUE USING INTELLIGENT TESTER (COOLANT TEMP)

Connect an intelligent tester to the DLC3.

Turn the ignition switch to ON.

Turn the tester on.

Enter the following menus: Powertrain / Engine and ECT / Data List / 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: 80°C to 100°C (176°C to 212°F).

REPLACE ENGINE COOLANT TEMPERATURE SENSOR (Click here)

OK

8.INSPECT MANIFOLD ABSOLUTE PRESSURE SENSOR

Connect the intelligent tester to the DLC3.

Turn the tester on.

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

Read the MAP value.
Standard:
Ignition switch ON:
80 to 110 kPa

REPLACE MANIFOLD ABSOLUTE PRESSURE SENSOR (Click here)

OK

9.CHECK FUEL PRESSURE

Check the fuel pressure (Click here).

REPAIR OR REPLACE FUEL SYSTEM

OK

10.CHECK EXHAUST GAS LEAK

OK:: No gas leak.

REPAIR OR REPLACE EXHAUST SYSTEM

OK

11.CHECK SPARK AND IGNITION

Perform a spark test.
CAUTION:
During the test, disconnect all of the fuel injector assembly connectors.
NOTICE:
Do not crank the engine for more than 2 seconds.
1. Remove the ignition coil from the cylinder head.
2. Install the spark plug onto the ignition coil.
3. Disconnect the all of the fuel injector assembly connectors.
4. Touch the spark plug to the cylinder head.
5. Crank the engine for less than 2 seconds and check for spark.
  OK:
  Sparks jump across electrode gap.
6. Reconnect the all of the fuel injector assembly connectors.
7. Reinstall the ignition coil.

HINT:

If the spark plugs or ignition system malfunctions, engine misfire may occur. The misfire count can be read using an intelligent tester. Enter the following menus: Powertrain / Engine and ECT / Data List / Cylinder #1 Misfire Count - Cylinder #3 Misfire Count.

REPAIR OR REPLACE IGNITION SYSTEM

OK

12.INSPECT FUEL INJECTOR ASSEMBLY (INJECTION AND VOLUME)

HINT:

Refer to the fuel injector inspection procedure (Click here).
If the fuel injectors malfunction, engine misfire may occur. The misfire count can be read using an intelligent tester. Enter the following menus: Powertrain / Engine and ECT / Data List / Cylinder #1 Misfire Count - Cylinder #3 Misfire Count.

REPLACE FUEL INJECTOR ASSEMBLY (Click here)

OK

13.INSPECT AIR FUEL RATIO SENSOR (HEATER RESISTANCE)

Disconnect the air fuel ratio sensor connector.

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) - 3 (A1A+)
Always
10 kΩ or higher
Text in Illustration
*1
Component without harness connected:
(Air Fuel Ratio Sensor)

Reconnect the air fuel ratio sensor connector.

REPLACE AIR FUEL RATIO SENSOR (Click here)

OK

14.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 (Check for Open):

Tester Connection	Condition	Specified Condition
B34-1 (HA1A) - B8-104 (HT1A)	Always	Below 1 Ω
B34-3 (A1A+) - B8-103 (A1A+)	Always	Below 1 Ω
B34-4 (A1A-) - B8-126 (A1A-)	Always	Below 1 Ω

Standard Resistance (Check for Short):

Tester Connection	Condition	Specified Condition
B34-1 (HA1A) or B8-104 (HT1A) - Body ground	Always	10 kΩ or higher
B34-3 (A1A+) or B8-103 (A1A+) - Body ground	Always	10 kΩ or higher
B34-4 (A1A-) or B8-126 (A1A-) - Body ground	Always	10 kΩ or higher

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

Reconnect the ECM connector.

Reconnect the air fuel ratio sensor connector.

REPAIR OR REPLACE HARNESS OR CONNECTOR

OK

15.REPLACE AIR FUEL RATIO SENSOR

Replace the air fuel ratio sensor (Click here).

NEXT

16.PERFORM CONFIRMATION DRIVING PATTERN

(a) Connect an intelligent tester to the DLC3.
(b) Turn the ignition switch to ON.
(c) Turn the tester on.
(d) Clear DTCs (Click here).
(e) Switch the ECM from normal mode to check mode using the tester (Click here).
(f) Start the engine and warm it up with all the accessories switched off.
(g) Drive the vehicle at between 60 km/h and 120 km/h (38 mph and 75 mph) and at an engine speed of between 1400 rpm and 3200 rpm for 3 to 5 minutes.
HINT:
If the system is still malfunctioning, the MIL will be illuminated during step (g).
NOTICE:
If the conditions in this test are not strictly followed, no malfunction will be detected.

NEXT

17.CHECK WHETHER DTC OUTPUT RECURS (DTC P0171 OR P0172)

Connect an 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 DTCs.

Result
Result	Proceed to
No output	A
P0171 or P0172	B

REPLACE ECM (Click here)

END

DTC P0171 System Too Lean (Bank 1)

DTC P0172 System Too Rich (Bank 1)

for Preparation Click here

DESCRIPTION

MONITOR DESCRIPTION

WIRING DIAGRAM

INSPECTION PROCEDURE