Fuel and air

P0171 — System Too Lean (Bank 1)

Severity: medium Drive with care, to a shop

What P0171 means

The engine computer constantly adjusts fuel delivery based on oxygen sensor feedback. P0171 sets when it has to add too much fuel (a high positive fuel trim) to keep Bank 1 balanced, meaning the mixture stays lean. The cause is usually extra unmetered air or too little fuel reaching the cylinders.

Symptoms

  • Check Engine light on
  • Rough or unstable idle
  • Hesitation or poor acceleration
  • Worse fuel economy
  • Possible misfires
  • Long cranking before start
  • Often appears with P0174 on V-type engines

Possible causes

  1. Vacuum or intake air leak: intake boot, vacuum hoses, intake manifold gasket
  2. Dirty or failing MAF sensor
  3. Low fuel pressure: weak pump, clogged filter or faulty regulator
  4. Faulty or slow upstream oxygen sensor
  5. Exhaust leak ahead of the upstream O2 sensor
  6. Leaking PCV system or brake booster hose
  7. Clogged or failing fuel injectors

How to diagnose

  1. Scan for codes and freeze-frame data; look for P0174 and misfire codes
  2. Read short-term and long-term fuel trims at idle and at about 2,500 rpm
  3. Inspect the intake tract, vacuum hoses and PCV lines for cracks and loose connections
  4. Smoke-test the intake or spray carb cleaner near joints to find leaks
  5. Check MAF readings and clean the sensor with MAF-safe cleaner
  6. Measure fuel pressure and compare with the factory spec
  7. Look for exhaust leaks upstream of the O2 sensor
  8. Check O2 sensor signal on a scan tool graph and compare injector balance

Common fixes

  • Repair the air leak: replace the hose, boot or manifold gasket
  • Clean or replace the MAF sensor
  • Replace the fuel filter, pump or pressure regulator
  • Replace the upstream oxygen sensor
  • Repair the exhaust leak
  • Clean or replace fuel injectors

P0171 on different brands

Ford · manuals
Ford's extended format P0171:00 means system too lean (Bank 1) with no sub-type; it describes the monitor condition detected, not a confirmed failed part.

Questions and answers

What does code P0171 mean?

The engine computer found the Bank 1 mixture too lean: too much air or too little fuel. Vacuum leaks, a dirty MAF sensor or weak fuel delivery are the most common causes.

Can I drive with P0171?

For a short time, yes, but get it fixed soon. A lean mixture can lead to misfires, poor economy and an overheated catalytic converter.

What should I check first for P0171?

Start with a visual check of intake boots and vacuum hoses for leaks, then look at MAF readings and fuel trims. These are the most common causes and the cheapest checks.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.