P0171 indicates the engine control module has detected a lean air-fuel mixture on Bank 1 (the engine bank containing cylinder #1). The oxygen sensor reports consistently low voltage, meaning excess air is entering the combustion chambers relative to fuel. The PCM attempts to compensate by adding fuel but cannot achieve the target mixture.
Unmetered air entering the intake manifold through cracked hoses, faulty PCV valve, damaged intake gasket, or throttle body issues causes excess air to bypass the MAF sensor.
Weak fuel pump, clogged fuel filter, or failing fuel pressure regulator cannot deliver adequate fuel to the injectors despite normal injector operation.
Contaminated mass air flow sensor provides inaccurate air flow readings to the PCM, causing incorrect fuel calculations.
Failed upstream O2 sensor may report consistently lean readings even when the mixture is correct, or fail to detect actual lean conditions.
Leaking exhaust manifold or header allows ambient air to enter the exhaust stream, causing false lean readings at the oxygen sensor.
Rarely, the engine control module itself may have a software or hardware fault causing incorrect fuel trims; this is the least likely cause and should be diagnosed only after all other possibilities are ruled out.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
Short distances at moderate load are generally safe, but the condition can cause catalytic converter damage over time and may leave you stranded if the lean condition worsens. Repair should be prompt.
No. The engine control module is almost never the cause of a lean code. The PCM may be replaced unnecessarily by some shops or DIYers who diagnose the problem incorrectly. Always rule out vacuum leaks, fuel delivery, and sensors before suspecting the computer.
Yes. A contaminated MAF sensor reads lower than actual airflow, causing the PCM to inject less fuel than needed. Cleaning with proper MAF sensor cleaner often resolves this.
No. P0171 is for Bank 1 (lean), while P0172 is for System Too Rich Bank 1. They indicate opposite conditions and have different causes.