P0171P0174

P0171 and P0174 are generic OBD-II diagnostic trouble codes indicating that the engine is running lean on Bank 1 and Bank 2, respectively. A lean condition means the air-fuel mixture has too much air relative to fuel, which is detected when the engine management computer sees fuel trims outside its expected range. These codes are often accompanied by a lit check engine light and may appear together on V‑engine configurations.

Common causes include vacuum leaks in the intake system (such as cracked hoses, the PCV valve, or intake manifold gaskets), a dirty or faulty mass air flow (MAF) sensor, a restricted air filter, or incorrect mass air readouts. Fuel delivery problems—low fuel pressure, weak fuel pump, clogged fuel filter, or dirty/injectors—can also produce a lean condition. Exhaust leaks before the oxygen sensors, faulty downstream O2 sensors, or incorrect sensor readings (MAP/MAF) can contribute. In forced induction engines, boost leaks or malfunctions can also trigger lean codes. Environmental factors like extreme intake temperatures or cold starts can influence fuel trims as well.

Diagnostic steps typically begin with reading live data and freeze-frame information, inspecting intake vacuum lines, and checking for proper MAF readings. A smoke test can reveal vacuum leaks, while verifying fuel pressure helps assess delivery. Clearing the codes after repairs and performing a road test confirms that fuel trims normalize and the condition does not recur. Repairs are tailored to the root cause and may include repairing leaks, cleaning or replacing the MAF sensor, fixing fuel delivery issues, or replacing faulty O2 sensors or related components.

Note that Bank 1 and Bank 2 refer to the two sides of a multi-bank engine; some four-cylinder engines have only Bank 1.