Code P0171: mixture too lean (bank 1)

Original definition (SAE J2012): System Too Lean (Bank 1)^[1][2]

• P0171System too lean (bank 1): the air-fuel mixture has too much air or too little fuel.
• P0174System too lean (bank 2): the same problem as P0171, but detected by the bank 2 lambda sensor.^[3][4]

1. 1Air leaks in the intake: cracked or loose vacuum hoses or sleeves, manifold gasket, or boost pipe on turbo engines. This is the most common cause: air that enters after the mass air flow sensor is not measured and the ECU cannot compensate.^{[1][2][7][8][9]}
2. 2Dirty or faulty mass air flow sensor (MAF), which measures the incoming air incorrectly.^{[1][2][7][8][9]}
3. 3Low fuel pressure: weak pump, clogged fuel filter or faulty regulator.^{[1][2][7][8][9]}
4. 4Clogged, contaminated or leaking injectors.^[1][7][9]
5. 5Faulty or stuck PCV valve (crankcase ventilation) or a broken hose.^[1][2][4][8]
6. 6EVAP purge valve stuck open, or leaks in that system.^[2][3][8]
7. 7Exhaust leaks upstream of the front lambda sensor, which give it false readings.^[1][8][9]
8. 8Faulty lambda sensor, which reports incorrectly; it is not usually the first suspect.^{[1][2][4][7][8]}
9. 9Causes cited by only one source: EGR valve stuck open, coolant temperature sensor with a wrong signal, blocked catalytic converter or a badly fitted aftermarket intake.^[2][8]

• Engine warning light on the instrument cluster.^[1][8]Engine warning light: faults that cause it
• Lack of power and slow acceleration.^[1][2][7]Less power: faults that cause it
• Rough idle.^[2][7][8]Vibrations: faults that cause it
• The engine may stall when the car comes to a stop.^[2][7][8]Engine stalls: faults that cause it
• Hesitation when accelerating.^[7]Jerking: faults that cause it
• Hard starting.^[2][8]Hard to start: faults that cause it
• Increased fuel consumption.^[1]Uses more fuel: faults that cause it
• Hissing or whistling in the engine bay when the cause is a vacuum leak.^[2]
• Knock (pinking) when accelerating under load or uphill.^[2]

1. 1Do a visual inspection in the engine bay: air filter box properly closed, intake hose without cracks, vacuum hoses connected and unbroken.^[2]
2. 2Read the code with a diagnostic tool and check whether any other associated codes are stored.^[8]
3. 3Watch the short-term and long-term fuel trims live, at idle and at higher revs. High positive values confirm the lean mixture (the sources give different thresholds: above +10% or +15-20%). If the trim rises a lot at idle and drops as revs rise, an air leak is most likely; if at idle it is near zero and rises at higher revs, suspect the mass air flow sensor, a blocked catalytic converter or low fuel pressure.^[2][5][7]
4. 4Find the air leak with a smoke test on the intake, including the boost section on turbo engines.^[4][8]
5. 5Clean or check the mass air flow sensor with a specific cleaner and check that its live values make sense.^[1][7][8]
6. 6Check the fuel pressure at idle and under load, and the condition of the fuel filter.^[1][4][7][8]
7. 7Check the PCV, EVAP purge and EGR valves, the injectors and any exhaust leaks upstream of the lambda sensor.^[1][8][9]
8. 8Leave the lambda sensor until last: replace it only if the other air and fuel checks come out normal.^[4][8]

• P0171 on BMW7
• P0171 on Nissan4