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

Solution

1. Preparation

• Gather tools and parts needed for the repair.
• Disconnect the battery to ensure safety while working on electrical components.

2. Inspect and Repair Vacuum Leaks

• Tools Required: Vacuum gauge, hose clamp pliers
• Inspect all vacuum hoses for cracks or disconnections.
• Replace any damaged hoses and ensure all connections are secure.

3. Fuel System Inspection

• Tools Required: Fuel pressure gauge, socket set
• Remove the fuel pump relay or fuse to relieve fuel pressure.
• Connect the fuel pressure gauge to the fuel rail and turn the ignition to the "on" position (do not start) to measure fuel pressure.
• Compare the reading to the manufacturer specifications (typically 50-60 psi); if below, check the fuel pump and replace if necessary.

4. MAF Sensor Cleaning/Replacement

• Tools Required: MAF sensor cleaner, screwdriver
• Locate the MAF sensor in the air intake duct.
• Disconnect the electrical connector and unscrew the sensor.
• Clean the sensor with MAF cleaner; do not touch the sensor elements.
• Reinstall the sensor and reconnect the electrical connector. Replace if cleaning does not resolve the issue.

5. Fuel Injector Service

• Tools Required: Fuel injector cleaning kit, socket set
• Remove the fuel rail to access fuel injectors.
• Inspect injectors for clogs or leaks; use a cleaning kit to flush them if necessary.
• Reinstall the fuel rail ensuring all injectors are seated properly.

6. Reassemble and Test

• Reinstall any components removed during the inspection.
• Reconnect the battery and start the engine.
• Monitor for any error codes or symptoms.