Fuel and air

P0171System too lean, bank 1

System Too Lean Bank 1

Get it diagnosed soon

Generic OBD-II code

What the computer is measuring

The engine wants roughly fourteen and a half parts air to one part fuel. The computer does not measure that ratio directly. It measures how much air is coming in, injects what it calculates as the matching amount of fuel, then reads the oxygen sensor in the exhaust to see whether it got it right.

When the exhaust comes back with more oxygen left over than expected, the mixture was lean. The computer compensates by lengthening the injector pulse, and it records how much extra it is adding as fuel trim. Small corrections are entirely normal. P0171 is set when the correction on bank 1 has been pushed near its limit and held there, meaning the computer has almost run out of authority to fix the problem itself.

That framing tells you what to look for. A lean condition is either air the computer did not know about, or fuel it expected to deliver and did not. Unmetered air entering after the mass air flow sensor is the first case and by far the most common. A tired pump, a clogged filter or restricted injectors are the second.

On a V6 or V8, whether the matching bank 2 code is also stored is the single fastest way to split those apart. Anything shared by both banks is upstream of the split. Anything on one bank alone is on that bank.

What you will notice

  • Steady check engine light.
  • A rough or hunting idle that smooths out once the engine is revved.
  • Hesitation or a flat spot when pulling away.
  • Worse fuel economy than usual.
  • On a bad leak, an audible hiss from the engine bay at idle.

Likely causes, most common first

Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.

  1. A vacuum leak downstream of the mass air flow sensor

    The most common answer by a wide margin. Cracked intake boots, perished PCV hoses, a leaking intake manifold gasket and a failed brake booster hose all do it.

  2. A dirty or failing mass air flow sensor

    A sensor reading low tells the computer less air is coming in than really is, so it injects too little fuel. Cleaning the element with proper MAF cleaner is cheap and often works.

  3. Low fuel pressure

    A weak pump, a clogged filter or a failing pressure regulator starves every cylinder. Usually worse under load rather than at idle.

  4. Dirty or partly blocked fuel injectors

    Restricted flow leans the mixture out. More likely on higher mileage direct injection engines.

  5. An exhaust leak ahead of the upstream oxygen sensor

    Air pulled in through a leaking manifold reaches the sensor and reads as a lean exhaust even though the mixture in the cylinder was correct.

What to check, in order

  1. Read the short and long term fuel trim values at idle and then at about 2500 rpm.

    Trim high at idle and close to normal at higher rpm is the vacuum leak pattern, because a fixed size leak matters less as total airflow rises. Trim high at both points to a fuel supply or MAF problem.

  2. Inspect every intake hose, the PCV system and the brake booster hose with the engine idling.

    Squeezing a cracked hose or briefly pinching a line and watching the idle change locates the leak without any special tools.

  3. Look at the mass air flow reading against what the engine should be pulling at idle and at 2500 rpm.

    A reading well below expected points at the sensor or a restriction ahead of it. A reading in range moves the search to fuel delivery.

  4. Check fuel pressure at idle and under load, against the specification for the engine.

    Pressure that holds correctly rules out the pump, filter and regulator.

  5. Check whether P0174 is also stored on a V engine.

    Both banks lean means something they share, so air, fuel pressure or the MAF. One bank lean only means a leak or an injector on that bank.

Where this one turns up most

  • Vehicles past about ten years old, where rubber intake and vacuum hoses have hardened and split.
  • Engines with plastic intake manifolds, where the gasket or the manifold itself cracks.

Codes that often show up alongside it

What a DriveDecode adapter can read

A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.

Reading P0171 on your own car

DriveDecode pulls P0171 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.

See how DriveDecode works

Last reviewed 2026-09-05. Definition sourced from the published standard.