Generic OBD2 code · Fuel and air metering

P0171: System Too Lean (Bank 1)

The engine computer is adding more fuel than it is allowed to on bank 1 and still sees a lean mixture, meaning there is too much air or too little fuel.

CodeP0171
SystemFuel and air metering
Typical urgencyModerate
Can I keep driving?Often drivable short term, but a lean engine can run poorly, misfire and run hotter, so diagnose it soon.

What does P0171 mean?

The ECM constantly adjusts fuel delivery using short term and long term fuel trims. Positive trim means it is adding fuel to correct a lean reading from the upstream oxygen sensor. When the total correction reaches the limit for your engine and the mixture is still lean, P0171 is stored.

Bank 1 is the side of the engine with cylinder 1. If you also have P0174 (bank 2 lean) on a V engine, the cause is more likely something both banks share, such as a MAF sensor reading low, a large intake leak or low fuel pressure.

Common symptoms

  • Check engine light on
  • Rough or unstable idle
  • Hesitation or stumbling on acceleration
  • Misfires, sometimes with P0300 or a cylinder specific misfire code
  • Hard starting in some cases
  • Sometimes no obvious symptoms at all

Common causes

The causes below are the ones commonly seen with P0171. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

  • Vacuum or intake air leaks after the MAF sensor, such as a cracked hose, PCV hose or intake gasket
  • A dirty or faulty mass air flow (MAF) sensor
  • Low fuel pressure from a weak pump, clogged filter or faulty regulator
  • Restricted or leaking fuel injectors
  • An exhaust leak ahead of the upstream O2 sensor
  • A faulty upstream oxygen or air fuel ratio sensor (less common than the causes above)

What to check with a scanner

  • Open Live data and watch short term and long term fuel trims for bank 1 at idle, then at about 2,500 rpm. Add short and long term together to get the total correction.
  • If the total correction is high at idle but drops a lot at higher rpm, a vacuum leak is a common cause. If it stays high at all speeds, look more at fuel delivery or MAF readings.
  • Watch the MAF reading and compare it with the expected value for your engine size, if you know it.
  • Check freeze frame data to see the temperature, load and rpm when the code set.

How to read and clear P0171 with OBD Master Pro

  1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
  2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
  3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
  4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0171 is listed and note any other codes, since they often explain the cause.
  5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
  6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0171 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0171.

When to see a mechanic

Get professional help if any of these apply:

  • The engine misfires, stalls or the check engine light flashes
  • You smell fuel or see a fuel leak (stop driving)
  • You cannot find a vacuum leak and need a smoke test
  • Fuel pressure needs to be measured with a gauge

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Brand notes

Patterns below come from published manufacturer service information. They do not mean every vehicle of that brand has this problem, and a bulletin only applies to the vehicles it lists.

Ford, Lincoln and Mercury (2000 to 2004 models)

Ford service bulletin TSB 04-17-4 covers lean codes P0171 and P0174 on many 2000 to 2004 Ford, Lincoln and Mercury models. It notes that oxygen sensors, MAF sensors and the PCM are not an effective repair when a vacuum leak is the cause, and tells technicians to inspect vacuum hoses and PCV elbows and compare fuel trims at idle and 2,500 rpm. It describes a total correction of around +20% or more at idle as typical of a vacuum leak, depending on calibration.

Source: Ford TSB 04-17-4: MIL on, system lean DTCs, service tips

P0171 FAQ

Is P0171 serious?

It is not usually an emergency, but a lean mixture can cause misfires and higher combustion temperatures over time. Find and fix the cause rather than just clearing the code.

Can a bad oxygen sensor cause P0171?

It can, but vacuum leaks, MAF sensor problems and low fuel pressure are more common causes. The upstream sensor is often just reporting a real lean condition.

What is the difference between P0171 and P0174?

P0171 is for bank 1 and P0174 is for bank 2. Seeing both at once on a V engine suggests a shared cause such as a MAF reading, an intake leak or fuel pressure.

Will cleaning the MAF sensor fix P0171?

Sometimes. A contaminated MAF sensor can under report airflow. Use a cleaner made for MAF sensors, then clear the code and watch fuel trims to see if they return toward normal.

Check P0171 on your own car

OBD Master Pro turns your Android phone and a generic ELM327 adapter into a code reader: read and clear codes, view freeze frame and live data, and check emissions readiness.

Get it on Google Play

Sources

Published . This guide is general information for generic SAE codes. Always follow the service information for your specific vehicle.