What causes P0171 system too lean bank 1?
Short answer: P0171 means the computer is adding more fuel than its limit to keep bank 1 at the right air-fuel ratio, usually from unmetered air, low fuel delivery or a skewed airflow reading.
The code sets when long-term fuel trim, or total trim, reaches the maker's lean threshold. The oxygen sensor is usually the messenger, not the cause. The three big families are unmetered air entering after the mass airflow sensor, not enough fuel reaching the cylinders, and an airflow sensor that underreports air.
Use fuel trims across operating conditions to sort it out. Trims that are high at idle and improve at 2500 rpm point toward a vacuum leak: cracked intake boots, PCV hoses, intake gaskets or brake booster lines. A smoke test is the fastest way to find those. Trims that worsen under load point toward fuel delivery, so check fuel pressure and volume, wearing safety glasses and keeping ignition sources away while the gauge is connected. Trims that are high across the board with low airflow readings for the engine size point toward a contaminated or failing mass airflow sensor.
If P0174 for bank 2 sets at the same time, look for something common to both banks rather than a single injector or gasket.
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General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.