P0171System Too Lean Bank 1
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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 learned base correction per load and speed range. Positive values mean fuel is being added because the mixture was lean, negative values mean fuel is being removed; around ±10 % is normal. P0171 means long-term trim on bank 1 is at its upper limit: the module already adds the maximum fuel and the mixture still stays lean. GM sets it at a long-term factor of about 1.30–1.36 (+30–36 %, engine-specific), Ford above +25 % with short-term trim above +1 %; it needs two trips and is suspended during heavy purge flow or intrusive tests. GM bulletins repeatedly trace it to leaking crankcase ventilation hoses or pipes on V6 and V8 engines; Ford found leaking direct injectors. P0171 together with P0174 points to a shared cause. Diagnosis hint: trims very high only at idle and normal under load point to an air leak; trims rising with load point to fuel shortage or an air flow sensor reading low. The oxygen sensor only reports what is in the exhaust and is rarely the cause.
- What you may notice
- 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)
- Likely causes
- LikelyAir leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line)
- LikelyCrankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick)
- PossibleAir flow sensor dirty or drifting (measures too little air)
- PossibleFuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty)
- RareInjectors clogged, or exhaust leak ahead of the oxygen sensor
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €20 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.