how to fix a P0171 code in my BMW 3 Series E90/E91?

Solution

1. Preparation

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

2. Inspect and Repair Vacuum Leaks

• Tools Required: Vacuum gauge, hose clamps, screwdriver.
• Inspect all vacuum hoses for cracks, damage, or disconnections.
• Replace any damaged hoses and ensure all connections are tight.

3. Clean or Replace MAF Sensor

• Tools Required: MAF cleaner spray, screwdriver.
• Locate the MAF sensor in the air intake duct.
• Disconnect the electrical connector and remove the sensor from its housing.
• Spray the MAF cleaner onto the sensor and allow it to dry before reinstalling.
• Reconnect the electrical connector and ensure it is secure.

4. Test Fuel Pressure

• Tools Required: Fuel pressure gauge.
• Attach the fuel pressure gauge to the fuel rail service port.
• Turn the ignition on (do not start the engine) and observe the pressure reading.
• If the pressure is below specifications, inspect the fuel pump and filter for issues.

5. Inspect Fuel Injectors

• Tools Required: Multimeter, injector cleaner.
• Disconnect the fuel injector connectors and test each injector's resistance with a multimeter.
• Clean the injectors using an injector cleaning service or replace if they are faulty.

6. Replace Faulty Oxygen Sensors (if necessary)

• Tools Required: Oxygen sensor socket, wrench.
• Locate the upstream oxygen sensor (before the catalytic converter).
• Disconnect the sensor and remove it using the oxygen sensor socket.
• Install the new sensor, ensuring it is tightened to manufacturer specifications (typically 30-40 Nm).
• Reconnect the electrical connector.