What causes the P0171 code to appear in a Toyota Corolla?

The P0171 code is a diagnostic trouble code that indicates a lean condition in the engine, where there is too much air in comparison to the amount of fuel. This code may appear in a Toyota Corolla due to the following reasons: a. Vacuum leaks: Leaks in the intake manifold, throttle body, or vacuum hoses can cause excess air to enter the engine, leading to a lean condition. b. Faulty mass airflow (MAF) sensor: A malfunctioning MAF sensor can provide incorrect air intake readings, causing the engine to run lean. c. Dirty or clogged fuel injectors: If the fuel injectors are not delivering the correct amount of fuel to the engine, a lean condition can occur. d. Faulty oxygen sensor: A malfunctioning oxygen sensor can provide incorrect fuel-to-air ratio readings, causing the engine to run lean. e. Low fuel pressure: A weak fuel pump, clogged fuel filter, or malfunctioning fuel pressure regulator can cause low fuel pressure, leading to a lean condition. f. Intake air leaks: Leaks in the air intake system, such as a cracked air intake hose, can cause excess air to enter the engine, resulting in a lean condition.