P0171: System Too Lean (Bank 1)

P0171 means System Too Lean (Bank 1). The engine computer constantly adjusts fuel delivery (fuel trim). This code means bank 1 (the side with cylinder 1) is running lean (too much air or too little fuel) and the computer has reached the limit of how much fuel it can add to correct it.

P0171 is very often caused by an unmetered air leak after the MAF sensor. On high-mileage engines, look closely at PCV hoses and intake gaskets. If P0174 is set too, the cause is probably shared by both banks.

CodeP0171
SystemFuel trim
SeverityModerate
TypeGeneric powertrain

Can I keep driving? Drivable with care. Diagnose soon: performance, economy or other components can be affected.

Common symptoms

  • Check engine light on
  • Hesitation or stumbling on acceleration
  • Rough idle
  • Possible misfires
  • Reduced power

Possible causes

  • Vacuum leak: cracked hose, intake gasket, PCV hose or brake booster hose
  • Dirty or faulty mass air flow (MAF) sensor under-reporting air
  • Low fuel pressure: weak pump, clogged filter or faulty regulator
  • Clogged or restricted fuel injectors
  • Exhaust leak ahead of the upstream oxygen sensor

How to diagnose P0171

  1. Read short- and long-term fuel trims at idle and at 2,500 RPM. A lean condition that improves at higher RPM points to a vacuum leak; one that gets worse under load points to fuel delivery or the MAF.
  2. Smoke-test the intake for vacuum leaks.
  3. Inspect and clean the MAF sensor with a dedicated MAF cleaner.
  4. Test fuel pressure and volume against specification.
  5. If both banks are lean (P0171 and P0174), look for a shared cause such as MAF, fuel pressure or a central vacuum leak.

What the code means, digit by digit

PPowertrain0Generic (SAE) code1Sub-system71Specific fault

Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.

Symptoms linked to P0171

Engine knocking or pingingRough idleHesitation or stumble when acceleratingPoor fuel economyCheck engine light onCar stalls at idle or when stopping

Related codes

General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.