what should I do if my BMW 3 Series E90/E91 (2005-2012) has a check engine light and P0171 code?

Solution

1. Preparation

• Gather tools and materials.
• Disconnect the battery to prevent electrical shorts.
• Ensure the engine is cool before starting work.

2. Inspect and Repair Air Intake System

• Sub-steps:
  1. Visually inspect all rubber hoses and connections for cracks or disconnections.
  2. Replace any damaged hoses or gaskets.
  3. Tighten all loose clamps and connections.

3. Clean or Replace MAF Sensor

• Tools Required: MAF cleaner, Torx screwdriver
• Sub-steps:
  1. Locate the MAF sensor in the air intake duct.
  2. Disconnect the electrical connector.
  3. Remove the MAF sensor using the Torx screwdriver.
  4. Clean the sensor with MAF cleaner, following the manufacturer's instructions.
  5. Reinstall the MAF sensor and reconnect the electrical connector.

4. Check Fuel System

• Tools Required: Fuel pressure gauge
• Sub-steps:
  1. Connect the fuel pressure gauge to the fuel rail.
  2. Turn the ignition to the "ON" position without starting the engine to check the fuel pressure.
  3. Compare the fuel pressure reading with the specifications in the service manual.
  4. If low, inspect the fuel pump and filter for issues, and replace as necessary.

5. Test Oxygen Sensors

• Sub-steps:
  1. Using a multimeter, test the voltage output of the upstream oxygen sensor at idle.
  2. If the sensor voltage is consistently below 0.1V or above 0.9V, replace the sensor.

6. Reconnect Battery and Clear Codes

• Reconnect the battery terminals securely.
• Use the OBD-II scanner to clear the P0171 code.