OBD2 code P0171: System Too Lean Bank 1
With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a lea
Detected Root Component
Powertrain Component
Observed Symptoms
- Check engin light on (after two drive cycles)
- Rough or unsteady idle, possibly stalling
- Hesitation or lack of power when accelerating
- Hissing or whistling from the engin bay (air leak, PCV)
Possible Causes
- Air leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line)
- Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick)
- Air flow sensor dirty or drifting (measures too little air)
- Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty)
- Injectors clogged, or exhaust leak ahead of the oxygen sensor
Tools Required
Interactive Diagnostic
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Parts likely to be the cause (Sonde O2 / Lambda)
Sonde O2 / Lambda
Photo d'illustration : Sonde O2 / Lambda
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Frequently asked questions about code P0171
What does code P0171 mean? ▾
OBD2 code P0171: System Too Lean Bank 1. With closed-loop control the engine module corrects the injected fuel using the oxygen sensor: short-term fuel trim reacts within fractions of a second, long-term trim stores a lea
What are the symptoms of code P0171? ▾
Code P0171 may cause: Check engine light on (after two drive cycles), Rough or unsteady idle, possibly stalling, Hesitation or lack of power when accelerating, Hissing or whistling from the engine bay (air leak, PCV). The intensity of these symptoms varies depending on the vehicle and driving conditions.
What causes code P0171? ▾
The most common causes of code P0171 are: Air leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line), Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick), Air flow sensor dirty or drifting (measures too little air), Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty). A proper OBD2 diagnostic will pinpoint the exact cause.
Is it serious to drive with code P0171? ▾
Code P0171 has moderate severity (Warning). Short-term driving remains possible, but a prompt check is recommended to prevent worsening.
How do I fix code P0171? ▾
Start by reading the code with an OBD2 scanner, then follow the interactive decision tree on this page. Priority causes to check: Air leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line), Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick), Air flow sensor dirty or drifting (measures too little air), Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty). Difficulty: Medium · Estimated time: 30-60min.