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Powertrain fault code

P0171: System Too Lean (Bank 1)

Fix soon

Drivable short-term, but a lean engine can misfire and run hot. Have it checked soon.

What P0171 means

The engine is getting too much air or too little fuel, and the computer has reached the limit of what it can correct.

Common causes

  • Vacuum or intake air leak (cracked hose, intake gasket)
  • Dirty MAF sensor
  • Weak fuel pump or clogged fuel filter
  • Leaking PCV valve or hose

What to check first

  1. Listen for hissing from vacuum hoses with the engine running.
  2. Check the fuel trim numbers on a live scan.
  3. Clean the MAF sensor with proper MAF cleaner.

P0171 on popular makes

P0171 on a Toyota

  • Most Toyotas use an air-fuel ratio sensor before the catalytic converter; a lean code is more often a dirty MAF or an intake leak than that sensor.
  • On V6 and V8 Toyotas, P0171 together with P0174 (both banks lean) points to something shared: the MAF, an intake or PCV leak, or low fuel pressure.

P0171 on a Chevrolet

  • Vacuum leaks are commonly reported on GM engines: intake manifold gaskets on older V6 engines and cracked PCV or vacuum hoses.
  • If P0174 is also stored, check shared causes first: MAF sensor, a vacuum leak, a weak fuel pump or a blocked fuel filter.

P0171 on a Ford

  • Cracked intake or PCV hoses and leaking intake manifold gaskets are commonly reported on Ford engines, along with a dirty MAF sensor.
  • With P0174 stored too, look at causes shared by both banks before replacing oxygen sensors.

P0171 on a Nissan

  • A dirty MAF sensor and cracks in the rubber intake boot after the air filter are common finds.
  • After repairs, an idle air volume learn helps the engine settle.

Get P0171 explained for your car: $2.99

  • P0171 explained in full for your make and model: meaning, how urgent, likely causes and what to check, in order.
  • A short personal note from us within 24 hours, or your money back.
  • Your own private report page you can save as a PDF or show your mechanic.

Read your car's codes yourself, free

Plug a cheap ELM327 OBD-II adapter into the port under the dashboard and open the scan tool: Chrome or Edge on a computer with a USB adapter, or Chrome on Android with a Bluetooth LE adapter. It reads and clears codes and shows emissions readiness. It does not work on iPhone.

Open the free scan tool

Need help with P0171?

Tell us your car and what it is doing with P0171. We reply by email with what to check first, in plain words. Free.

Related codes

P0174

General guidance only, not a diagnosis. A qualified mechanic should confirm the cause before you replace parts.