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P0171 System Too Lean (Bank 1)

The engine is getting more air than the computer accounts for, and it has run out of room to add fuel to compensate — usually a vacuum leak or a dirty air sensor.

Fuel & air

The ECU trims fuel constantly to hold a correct air/fuel ratio, guided by the upstream oxygen sensor. When the oxygen sensor keeps reporting lean, the ECU adds fuel — and it can only add so much. P0171 means it hit that ceiling on bank 1 and still couldn't get there. The code is not itself a fault; it is the computer telling you it has exhausted its ability to hide one. Two families of cause produce it: unmetered air getting in, or fuel not getting in.

Diagnose P0171 on your specific vehicle Add your year, make and model for ranked causes, repair videos and the exact parts you need

Can I keep driving?

Driveable

Yes, usually with rough idle and some hesitation. Avoid sustained heavy load — that's where lean running does its damage.

How serious

Moderate. A lean mixture burns hotter, which over time damages valves, pistons and the catalytic converter. It rarely strands you, but it is not one to leave for a year.

Symptoms you'll notice

What causes P0171

Ordered by what to check first — how likely each cause is, weighed against what it costs to rule out. Working down the list in order is what stops you replacing an expensive part to fix a cheap problem.

01

Vacuum leak

The number one cause. Air entering downstream of the mass airflow sensor is air the ECU never counted. Cracked intake boots, brittle PCV hoses, a perished intake manifold gasket, a leaking brake booster hose. Rubber goes hard with heat and age, so this is very much a high-mileage fault. The tell is that the lean condition is worst at idle, where a fixed leak is a large fraction of total airflow, and improves at higher RPM.

02

Dirty or failing mass airflow sensor

A MAF coated in oil film — often from an over-oiled aftermarket air filter — under-reports airflow, so the ECU injects too little fuel. Cleaning the sensor element with proper MAF cleaner is cheap and frequently fixes it outright. Never touch the element with anything physical.

03

Weak fuel delivery

A tired fuel pump, a clogged filter, or a failing pressure regulator leaves the engine genuinely short of fuel. The signature is the opposite of a vacuum leak's: fine at idle, lean under load and acceleration when demand is highest.

04

Clogged fuel injectors

Deposits restrict flow across the bank. More common on high-mileage and direct-injection engines.

05

Failing upstream oxygen sensor

A lazy or biased sensor can report lean when the mixture is fine, and the ECU dutifully adds fuel it doesn't need. Worth suspecting when fuel trims are high but the engine runs and smells rich.

06

Exhaust leak ahead of the oxygen sensor

A cracked manifold or leaking gasket upstream of the sensor lets outside air get pulled in during exhaust pulses, which reads as lean even though the mixture entering the cylinder was correct.

07

Stuck-open PCV valve

The PCV system is a metered vacuum leak by design. When the valve sticks open it becomes an unmetered one — a cheap part and an easy check.

How to diagnose it

  1. Read short- and long-term fuel trims at idle and at around 2500 RPM. A big positive trim at idle that shrinks as RPM rises means a vacuum leak; a trim that grows with load means fuel delivery.
  2. Listen for hissing around the intake with the engine idling. Spraying a little carb cleaner near suspect joints and listening for an idle change works, but keep it away from hot surfaces.
  3. A smoke test of the intake is the definitive vacuum-leak hunt if you can borrow a smoke machine.
  4. Inspect all intake boots, PCV hoses and the brake booster hose for cracks — flex them by hand, since cracks often open only under movement.
  5. Clean the MAF sensor with dedicated MAF cleaner and re-test. Cheap, quick, and often the answer.
  6. Test fuel pressure at the rail, at idle and under load, against spec.
  7. Check for exhaust leaks upstream of the oxygen sensor.

Parts you might need

Tools for the job

Related trouble codes

Get the parts list for your vehicle Ranked causes, tutorials and live part prices for your exact year, make and model

Fitment matters. Causes and parts above are the common cases for P0171 across vehicles. The specific part numbers, procedures and torque specs vary by year, make, model and engine — always confirm against service data for your vehicle before buying parts or starting work.