P0171 code meaning
System Too Lean Bank 1
Powertrain system Generic code P01xx codes Check engine light
What P0171 means
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.
P0171 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P0171
- 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)
Common causes of P0171
- Most likelyAir leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line)
- Most 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)
- Less likelyInjectors clogged, or exhaust leak ahead of the oxygen sensor
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Oxygen sensor bank1 sensor1Mass air flow sensorFuel systemIntake manifold
What each part of P0171 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 1 | Area: fuel and air metering. |
| 71 | The exact fault in that area. |
What to do next
- Note P0171 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0171
What does P0171 mean?
P0171 means "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 learned base correction per load and speed range. It is a generic code, so it means the same on every make.
Is it safe to drive with P0171?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P0171 fail an emissions test?
Yes, it can. P0171 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0171 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P0171 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.