how to diagnose P0171 code on Toyota Highlander Third Generation (2014-2019)

Solution

1. Preparation

• Gather necessary tools and parts.
• Disconnect the battery to prevent electrical shorts.
• Ensure the vehicle is on a level surface with the engine cool.

2. Inspect and Repair Vacuum Leaks

• Sub-steps:
  1. Visually inspect all vacuum hoses and connections for cracks or disconnections.
  2. Use a smoke machine or carburetor cleaner to detect vacuum leaks at intake manifolds and hoses.
  3. Replace any damaged hoses or gaskets identified during inspection.

3. Clean or Replace the MAF Sensor

• Sub-steps:
  1. Disconnect the MAF sensor electrical connector.
  2. Remove the MAF sensor from the air intake.
  3. Clean the MAF sensor using MAF cleaner spray; do not touch the sensor elements.
  4. Reinstall the MAF sensor and reconnect the electrical connector. If it is damaged, replace it.

4. Check Fuel Pressure

• Sub-steps:
  1. Locate the fuel pressure test port on the fuel rail.
  2. Connect a fuel pressure gauge to the test port.
  3. Turn the ignition to the "ON" position (do not start the engine) and observe the pressure reading; it should meet manufacturer specifications (around 50-60 psi).
  4. If the pressure is low, check the fuel pump and filter for issues; replace if necessary.

5. Test and Clean Fuel Injectors

• Sub-steps:
  1. Remove the fuel injectors from the fuel rail.
  2. Use a fuel injector cleaner and tester to check each injector’s spray pattern and operation.
  3. Clean or replace injectors as necessary based on test results.

6. Inspect and Test Oxygen Sensors

• Sub-steps:
  1. Locate the upstream oxygen sensor (before the catalytic converter).
  2. Disconnect the sensor and use a multimeter to test its voltage output while the engine is running.
  3. If the sensor does not provide the correct voltage (usually between 0.1 to 0.9 volts), replace it.