Powertrain, generic · Fuel and air metering

P0171 System Too Lean Bank 1

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely 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

Symptoms

  • 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)

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–600
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

What to check first

  1. Check long-term fuel trim at idle and at 2500 rpm
  2. Smoke-test the intake for leaks
  3. Clean or test the MAF
  4. Measure fuel pressure

Parts involved

oxygen sensor bank1 sensor1mass air flow sensorfuel systemintake manifold

Related codes

P0174P0172P0170P0101P2187P2177

References: law.cornell.edu, gsi.ext.gm.com, gsi.ext.gm.com, fordservicecontent.com.