2000 Toyota 4runner P0171

Hello I'm currently working on finding the cause of this code. I believe that knowing the proper values for these sensors would help me pinpoint the problem.

*oxygen sensor bank 1 sensor 2 is reading 0-1v, which I believe is functioning as it should. Please correct me if I'm wrong.

*O2FT / Oxygen Fuel Trim bank 1 sensor 2 reads 99.15% at what I what I believe is at idle(I as of now only have information to work with now, not the physical scanner and vehicle.

*AF FT / Air Fuel Fuel Trim bank 1 sensor 1 reads a value of 1 with no unit. I believe this is the fault of the scanner not being completely compatible with the 4runner. If you know what the proper vale should be please let me know.

* AFS / Air Fuel Ratio Sensor bank 1 sensor 1 reads between 2.8 - 3.8v which I also believe is the proper value for this sensor.

I plan to use these values to help me go over these steps presented in another forum.

1999 Toyota 4Runner 3.4L, Eng Cfg V6, Eng Des 5VZFE, USA/Canada

Hotline Archive
With OEM Direct diagrams, components, TSBs, and R&R procedures

Number: 482996
Vehicle Application:
1998 4Runner 3.4 1999 Tacoma 3.4
1999 4Runner 3.4 2000 Tacoma 3.4
2000 4Runner 3.4 2000 Tundra 3.4

Customer Concern:
Has a check engine light on and a code P0171.

Tests/Procedures:
1. Monitor the fuel trim readings at idle and at 55 MPH driving down the road. If the numbers are low at idle but climb on a part throttle cruise, the airflow meter could be reading low or it may also have low fuel pressure. The mass air flow sensor grams per second should be 3.3-4.7 at idle and 12.9-18.3 at 2500 RPMs no load, if the readings are low the sensor could be bad.

2. Monitor the calculated load reading on a wide-open throttle acceleration. The reading should go to 85% or more. If not, then the airflow meter could be faulty.

3. Monitor the oxygen sensor inputs, both front and rear oxygen sensor signals. The oxygen sensor should not stay high or low voltage for extended periods of time. If the oxygen sensor shows low voltage and the fuel trim readings climb, check the performance of the oxygen sensor.

4. Check the vehicle for a vacuum leak by spraying carburetor spray around the intake manifold while watching the fuel trim readings. If the readings change, there is a vacuum leak.
Tech Tips:
If fuel trim readings are high at idle and go down on a free rev, there is a vacuum leak. If the fuel trim readings are high at idle and at cruise, suspect fuel injectors.


Diagnostic Codes
P0171

Confirmed Fix
225 - Mass Air Flow (MAF) Sensor
9 - Engine Air Filter, Mass Air Flow (MAF) Sensor
4 - Oxygen (O2) Sensor(s)
2 - Fuel Pump
2 - Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2 (B1S2)
2 - Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1 (B1S1)
1 - Front Oxygen (O2) Sensor
1 - Vacuum Leak
1 - Fuel Injector(s), Mass Air Flow (MAF) Sensor
1 - Cruise Control Vacuum Line
1 - Intake Manifold, Mass Air Flow (MAF) Sensor
1 - Mass Air Flow (MAF) Sensor, Throttle Body Cleaning Procedure
1 - Mass Air Flow (MAF) Sensor, Mass Air Flow (MAF) Sensor O-Ring
1 - Exhaust Pipe, Mass Air Flow (MAF) Sensor
1 - Air Intake Tube
1 - Mass Air Flow (MAF) Sensor, Throttle Body
1 - Air Fuel Ratio (AFR) Sensor
1 - Engine Vacuum
1 - Oxygen (O2) Sensor, Bank 1 Sensor 2