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

Solution

1. Preparation

• Gather tools and parts required.
• Ensure the vehicle is parked on a level surface and the engine is cool.

2. Inspect and Repair Vacuum Leaks

• Sub-steps:
  1. Visually inspect all vacuum hoses connected to the intake manifold for cracks or disconnections.
  2. Use a smoke machine or carburetor cleaner to detect leaks.
  3. Replace any damaged hoses or gaskets as necessary.

3. Clean or Replace Mass Air Flow (MAF) Sensor

• Tools Required: MAF sensor cleaner
• Sub-steps:
  1. Disconnect the MAF sensor electrical connector.
  2. Remove the MAF sensor from the air intake duct.
  3. Spray the MAF sensor with cleaner and allow it to dry completely.
  4. Reinstall the MAF sensor and reconnect the electrical connector.

4. Check Fuel Pressure

• Tools Required: Fuel pressure gauge
• Sub-steps:
  1. Locate the fuel rail and connect the fuel pressure gauge.
  2. Turn the ignition to the "ON" position without starting the engine, and check for the specified fuel pressure (refer to the service manual for specifications).
  3. If the pressure is low, inspect the fuel pump and fuel filter, replacing them if necessary.

5. Inspect Oxygen Sensors

• Sub-steps:
  1. Locate the upstream and downstream oxygen sensors.
  2. Check the wiring and connectors for damage or corrosion.
  3. Test the sensors with a multimeter to ensure proper voltage output; replace if defective.

6. Clear Codes and Test Drive

• Sub-steps:
  1. Use the OBD-II scanner to clear the codes.
  2. Test drive the vehicle under various conditions to ensure the check engine light does not reappear.