Several things can cause this:
* Mass Airflow Sensor (MAF) issues: A faulty MAF sensor provides inaccurate readings to the engine control unit (ECU), leading to a lean condition. This is a very common cause. It might be dirty, malfunctioning, or even disconnected.
* Vacuum Leaks: Leaks in the intake manifold, vacuum hoses, or other parts of the intake system allow unmetered air to enter the engine, upsetting the air/fuel ratio.
* Fuel Injector Problems: A clogged or malfunctioning fuel injector in bank 1 can't deliver enough fuel, resulting in a lean mixture. This could be due to a clogged injector or low fuel pressure.
* Fuel Pump Issues: A weak or failing fuel pump may not be able to supply enough fuel pressure to the injectors.
* Fuel Pressure Regulator Problems: A faulty fuel pressure regulator may not maintain the correct fuel pressure, leading to insufficient fuel delivery.
* EGR Valve Problems: A stuck-open EGR (exhaust gas recirculation) valve can dilute the air/fuel mixture, potentially causing a lean condition (though this is less common for a P0171 specifically). A faulty EGR valve can cause a lean mixture under certain conditions.
* Oxygen Sensor (O2 Sensor) Issues: While less likely to be the *direct* cause of a P0171, a faulty O2 sensor in bank 1 *can* prevent the ECU from accurately correcting a lean condition, exacerbating the problem and causing the code to set.
* PCV Valve Issues: A clogged PCV (positive crankcase ventilation) valve can cause a vacuum leak or affect the air/fuel mixture.
Troubleshooting Steps:
Diagnosing a P0171 requires a systematic approach. It's generally recommended to:
1. Check for obvious vacuum leaks: Visually inspect all vacuum hoses and connections in bank 1 for cracks or loose connections.
2. Clean the MAF sensor: Carefully clean the MAF sensor with MAF sensor cleaner (following the instructions on the cleaner).
3. Check fuel pressure: Test the fuel pressure to ensure it's within specifications.
4. Inspect fuel injectors: This may require a professional diagnostic scan and potentially injector flow testing.
5. Check the O2 sensor(s): Ensure they are functioning correctly.
A professional mechanic with diagnostic tools can perform more thorough testing to pinpoint the exact cause of the lean condition. Simply replacing parts without a proper diagnosis is often inefficient and costly.
