A car runs lean when the air-to-fuel ratio in the combustion chamber is too high, meaning there's too much air and not enough gasoline for a complete burn. This is often caused by unmetered air entering the engine, a problem with the fuel delivery system, or faulty sensor data being sent to the car's computer (ECU).
The most frequent culprit is a vacuum leak. Your engine has a precise network of hoses that can crack or become disconnected over time. This allows air to sneak in after it has already been measured by the Mass Airflow (MAF) sensor. The ECU, unaware of this extra air, doesn't inject enough fuel, resulting in a lean condition. Symptoms include a rough idle, hesitation during acceleration, and the illuminated Check Engine Light with codes like P0171 or P0174.
Fuel delivery issues are another major cause. A weak fuel pump, a clogged fuel filter, or dirty fuel injectors can't supply the necessary volume of gasoline. Even if the ECU commands the correct amount of fuel, it never reaches the cylinders. A faulty oxygen (O2) sensor can also be to blame. If this sensor provides an incorrect "lean" reading to the ECU, the computer will continuously try to add more fuel than needed, which can actually mask the problem until it becomes severe.
Diagnosing the root cause requires a systematic approach, often starting with a smoke test to find vacuum leaks and checking fuel pressure.
