EngineLightCodes
Powertrain code

P0171 System Too Lean Bank 1

Severity
Fix soon
Typical repair cost
$20–$660
Do it yourself
Possible, medium
Shop time
0.5–4 hours

The engine on bank 1 is getting too much air or too little fuel. The usual suspect is an air leak after the mass airflow sensor, such as a cracked vacuum hose, but a dirty airflow sensor or weak fuel pump can cause it too.

How the engine computer 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.

Written from public technical sources and not yet checked by a mechanic. How we write these pages

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)

Common causes

  1. Air leak after the air flow sensor (intake manifold gasket, vacuum hoses, throttle body gasket, brake booster line) Most likely
  2. Crankcase ventilation leaking or faulty (PCV hose, diaphragm in valve cover, oil cap, dipstick) Most likely
  3. Air flow sensor dirty or drifting (measures too little air) Possible
  4. Fuel pressure or delivery too low (weak pump, clogged filter, regulator, tank nearly empty) Possible
  5. Injectors clogged, or exhaust leak ahead of the oxygen sensor Less likely

How to diagnose P0171

  1. Check whether P0174 is also set. If both banks are lean, look for causes they share, like the airflow sensor or fuel pressure.
  2. With the engine idling, listen for hissing and inspect vacuum hoses, the intake boot and the PCV hose.
  3. Watch long-term fuel trim on a scanner. If it drops back toward zero at higher rpm, it's a vacuum leak; if it gets worse, suspect fuel delivery.
  4. Clean the mass airflow sensor with MAF cleaner spray and recheck.
  5. Have fuel pressure tested if the leaks and sensor check out.

Vehicles that report P0171 most

VehicleYearsReportsMedian miles
Ford Windstar1999–20031982k
Chevrolet Cruze2011–20161063k
Chevrolet Sonic2012–2014680k
Chevrolet Traverse2013–20205—
Ford Focus2012–20185—
Ford F-1502003–20224—
Toyota Corolla2005–2018498k
Chevrolet Equinox2006–20244—
Toyota Camry2007–20194—
Toyota RAV42007–20224—

Counts of owner complaints to the NHTSA that mention P0171 (162 in total), not failure rates: popular models get more complaints simply because more of them are on the road. Models with fewer than 3 complaints are left out.

Reading the code

P
Powertrain
0
Generic (SAE) code
1
Fuel and air metering
71
Specific fault

Questions drivers ask

Can I drive with code P0171?

Yes, for now. You may notice rough idle or hesitation. Over time a lean engine can misfire and run hotter, so don't leave it for months.

Will my car pass an emissions inspection with P0171?

Probably not in states that test vehicles with an OBD-II scan: an emissions-related code that keeps the check engine light on is usually an automatic fail. Rules differ by state and county, so check yours.

Will P0171 clear by itself?

The light can go off after several drive cycles without the fault, but the code returns if the cause is still there. After a repair, the readiness monitors need a full drive cycle before an inspection.

Can I fix P0171 myself?

Often yes, with an OBD-II scanner and basic tools. Difficulty: medium. Start with the cheapest checks before replacing expensive parts.

Often reported together

Codes that owners mention in the same NHTSA complaints as P0171. They can point to a shared cause, so it's worth checking them together.

This page is general information, not a substitute for a professional diagnosis. Repair costs are rough estimates for parts and labor and vary by vehicle and region.