how to fix P0171 code in BMW 3 Series E90/E91 (2005-2012)

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Disconnect the negative battery terminal to prevent electrical shorts.
• Allow the engine to cool before starting work.

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 to detect hard-to-find leaks.
  3. Replace any damaged hoses or gaskets as needed.
• Tools Required: Smoke machine, pliers, replacement hoses or gaskets.

3. Clean or Replace MAF Sensor

• Sub-steps:
  1. Disconnect the MAF sensor connector and remove the sensor from the air intake duct.
  2. Clean the sensor using MAF sensor cleaner; do not use any other cleaning agents.
  3. Reinstall the MAF sensor and reconnect the electrical connector. If cleaning does not improve performance, replace the MAF sensor.
• Tools Required: MAF sensor cleaner, screwdriver.
• Safety Warning: Handle the MAF sensor carefully to avoid damage.

4. Check and Adjust Fuel Pressure

• Sub-steps:
  1. Connect a fuel pressure gauge to the fuel rail test port.
  2. Turn the ignition to the "On" position (do not start) to check the fuel pressure.
  3. Compare the reading to manufacturer specifications (typically around 50-60 psi for BMW).
  4. If pressure is low, check for a faulty fuel pump or clogged fuel filter and replace as necessary.
• Tools Required: Fuel pressure gauge, replacement fuel pump or filter if needed.

5. Inspect and Service Fuel Injectors

• Sub-steps:
  1. Remove the fuel rail to access the fuel injectors.
  2. Inspect each injector for signs of clogging or leaks; clean or replace as necessary.
  3. Reinstall the fuel rail and ensure all connections are secure.
• Tools Required: Injector cleaning kit, replacement injectors if needed.