my Volkswagen Golf MK5 (2004-2008) has a check engine light with P0171 code

Solution

1. Preparation

• Gather all tools and parts required for the repair.
• Disconnect the negative battery terminal to prevent electrical shorts.

Tools Required:

• OBD-II scanner
• Screwdrivers
• Socket set
• Torque wrench
• Multimeter
• Fuel pressure gauge

2. Inspect and Repair Vacuum Leaks

• Sub-steps:
  1. Visually inspect all vacuum hoses for cracks or disconnections.
  2. Use a smoke machine or carburetor cleaner to identify leaks by spraying around connections while the engine is running.
  3. Replace any damaged hoses or gaskets found during inspection.

3. Clean or Replace MAF Sensor

• Sub-steps:
  1. Locate the MAF sensor in the intake duct.
  2. Disconnect the electrical connector and remove the sensor using the appropriate socket.
  3. Clean the MAF sensor with MAF cleaner or replace it if damaged.
  4. Reinstall the sensor and reconnect the electrical connector.

4. Check Fuel System Pressure

• Sub-steps:
  1. Attach the fuel pressure gauge to the fuel rail’s test port.
  2. Turn the ignition to the "ON" position without starting the engine.
  3. Check that the fuel pressure is within the manufacturer's specifications (typically around 50-60 psi).
  4. If pressure is low, inspect the fuel pump and filter for issues.

5. Verify Oxygen Sensor Functionality

• Sub-steps:
  1. Locate the upstream oxygen sensor (pre-catalytic converter).
  2. Use a multimeter to test the sensor's voltage output while the engine is running.
  3. If the output is out of range (usually 0.1-0.9 volts), replace the sensor.

6. Clean Throttle Body

• Sub-steps:
  1. Remove the air intake duct connected to the throttle body.
  2. Use throttle body cleaner and a soft cloth to clean the interior of the throttle body.
  3. Reinstall the air duct securely.