P0171 Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

System too lean, bank 1

Monitor

If the engine runs normally, a lean code is an emissions and efficiency fault you can drive on briefly while booking a diagnosis; it should not be left, because the lean condition can worsen into misfire and converter damage.

It gets more serious if:

  • Inspect soon: Inspect soon if the engine idles roughly, hesitates, stalls, misfires or pings under load; a mixture that far off stresses the catalytic converter and can leave you stranded.
  • Stop: Stop if the check engine light starts flashing: that means an active misfire hot enough to damage the catalytic converter. Pull over where nothing flammable is under the car and let it cool.

Source: how we rate severity

P0171 in short

P0171 means the engine computer is adding as much extra fuel as it is allowed on bank 1 and the mixture is still lean. The usual cause is unmeasured air from a vacuum or crankcase-vent leak, then a dirty airflow sensor or weak fuel delivery. Get it checked soon if the engine hesitates, idles roughly or stalls.

The upstream oxygen sensor tells the computer whether the exhaust is rich or lean, and the computer corrects by adding or removing fuel. That correction is called fuel trim. P0171 sets on bank 1 (the side with cylinder 1; an inline engine has only bank 1) when the computer has been adding fuel up to its calibrated limit and still can't bring the mixture back.

The code says the mixture looks lean; it doesn't name a part. Most often extra air is getting in that the airflow sensor never measured, through a split intake boot, a cracked vacuum hose or a leaking crankcase ventilation hose. Less often the airflow sensor under-reads, the fuel pump or filter can't keep up, or an exhaust leak lets air reach the sensor and fool it.

How the trims behave narrows it down. A leak usually shows up most at idle and fades as engine speed rises, while a fuel-supply or airflow-sensor problem tends to stay or worsen under load. If both banks are lean, look first at something they share.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

  1. Vacuum leak: split intake boot, cracked hose or leaking intake gasket

    Common general cause
    Why it ranks here
    Air that sneaks in after the airflow sensor is never measured, so the engine runs lean. It is the most common lean cause and cheap to find with a smoke test, so it is checked first.
    What would change this
    Fuel trims that are high at idle and improve as engine speed rises point here; a smoke test that shows the leak confirms it. Trims that stay high or get worse under load move other causes up.
    How to test the vacuum hoses and intake leaks

  2. Leaking crankcase ventilation (PCV) valve, hose or oil-filler seal

    Common general cause
    Why it ranks here
    The crankcase ventilation system connects to the intake, so a split hose, failed valve or loose oil cap is a vacuum leak. Saab's own procedure checks it before anything else.
    What would change this
    A cracked PCV hose, whistling at the oil filler or a change in idle when the cap is loosened raises this; a sealed system lowers it.
    How to test the vacuum hoses and intake leaks

  3. Dirty or skewed mass airflow (MAF) sensor

    Common general cause
    Why it ranks here
    If the MAF under-reports airflow, the computer injects too little fuel, typically on both banks at once. Contamination, a wrong replacement part or air bypassing the sensor all do this.
    What would change this
    Airflow readings below known-good values for the engine, or trims that change after an approved cleaning, raise this; readings in range lower it.
    How to test the mass airflow (maf) sensor

  4. Weak fuel pump, clogged filter or faulty pressure regulator

    Common general cause
    Why it ranks here
    Too little fuel pressure or volume leaves the engine lean, often worst under load or at higher speed. It ranks below air leaks because it needs a gauge to prove.
    What would change this
    Fuel pressure or volume below specification during the conditions that set the code confirms it; pressure in specification rules it out.
    How to test the fuel pump

  5. Exhaust leak ahead of the bank 1 upstream oxygen sensor

    Common general cause
    Why it ranks here
    Outside air drawn in at a cracked manifold or gasket makes the sensor read lean even when the mixture is fine, so the computer adds fuel it doesn't need.
    What would change this
    Ticking at cold start, soot at the manifold or a leak found by smoke testing on that bank raises this; a sealed exhaust ahead of the sensor rules it out.

  6. EVAP purge valve stuck open

    Possible — needs testing
    Why it ranks here
    A purge valve that never closes feeds vapour and air into the intake at idle. It is less common, but quick to check by pinching or plugging the purge line.
    What would change this
    Trims that drop toward normal when the purge line is blocked confirm it; no change rules it out.
    How to test the evap canister purge valve

  7. Restricted or under-delivering fuel injectors on bank 1

    Possible — needs testing
    Why it ranks here
    Clogged or worn injectors deliver less fuel than commanded. A shop can compare injector balance and cylinder contribution, but injectors are not replaced just because the code is stored.
    What would change this
    One cylinder lagging on an injector balance or contribution test raises this; even results lower it.
    How to test the fuel injector

  8. Slow or biased bank 1 upstream oxygen or air-fuel sensor

    Possible — needs testing
    Why it ranks here
    A sensor that reads falsely lean makes the computer add fuel. Ranked last because a correctly responding sensor is usually reporting a real lean condition.
    What would change this
    A sensor that responds slowly or stays lean during a controlled rich change raises this; crisp response rules it out.
    How to test the oxygen (o2) sensor

What to test first

Start at step 1. Each result tells you what it means and where to go next.

  1. Record all codes and freeze-frame data, then compare long-term and short-term fuel trims on both banks at idle and at a steady 2,500 rpm on a warm engine

    Low riskTool: scan tool with live data

    Warning:Keep hands, hair and loose clothing clear of the belts and fan while the engine runs, and never run the engine in a closed garage.

    • IfTrims are high at idle and come down toward normal at 2,500 rpm

      It meansClassic unmetered-air pattern; look for a vacuum leak.

      Go to step 2

    • IfTrims stay high or rise as engine speed and load increase

      It meansAirflow measurement or fuel delivery is more likely.

      Go to step 3

    • IfOnly bank 1 is lean on a V-type engine

      It meansLook for something specific to that bank: an exhaust leak ahead of its sensor, an injector or the sensor itself.

      Go to step 5

  2. Inspect the intake boot, vacuum hoses, PCV hose and oil-filler seal, then have the intake smoke-tested. Briefly block the EVAP purge line and watch the trims

    Use cautionTool: smoke machine (shop); visual inspection

    Warning:Keep hands, hair and loose clothing clear of the belts and fan while the engine runs, and never run the engine in a closed garage.

  3. Check the airflow (MAF) reading against known-good values for your engine, and inspect the duct between the MAF and the throttle for cracks or loose clamps

    Low riskTool: scan tool with live data

  4. Have fuel pressure, and volume where specified, measured under the conditions that set the code

    Best left to a shopTool: fuel pressure gauge (shop)

    Warning:The fuel system is under pressure and gasoline is a fire risk. Fuel-pressure testing means opening the fuel system: relieve pressure first, no flames or sparks nearby, and leave it to a shop if you don't have the gauge and procedure for your engine.

  5. Check the exhaust ahead of the bank 1 upstream sensor for leaks, test that sensor's response to a controlled rich and lean change, and compare injector balance

    Best left to a shopTool: scan tool; smoke machine; injector balance test (shop)

    Warning:Exhaust parts stay hot enough to burn skin long after the engine stops. Let the car cool for at least an hour, and never get under a car held up only by a jack.

Symptoms

  • Check engine light with no change in how the car drives
  • Rough or hunting idle, hesitation, or stalling Gets more serious: stop and have it checked.
  • Pinging or spark knock when accelerating Gets more serious: stop and have it checked.
  • Hissing from the engine bay at idle
  • Check engine light flashing, or the engine shakes and stumbles under load Gets more serious: stop and have it checked.

Compare with: Rough idle with the check engine light on, Check engine light with loss of power

How to fix

  1. With the engine off, look over the intake duct between the air filter box and the throttle for splits and loose clamps, and check the vacuum and crankcase ventilation hoses for cracks or disconnected ends.

  2. Make sure the oil filler cap and dipstick are fully seated; on many engines a loose cap is a crankcase vacuum leak.

  3. If the airflow sensor is dirty and your maker allows cleaning, spray only the sensing element with dedicated MAF sensor cleaner, never touch it, and let it dry fully before reconnecting.

    Warning:Disconnect the sensor before cleaning, use only cleaner made for MAF sensors, and do not refit it wet.

Parts and cost context

  • Vacuum hoses and intake leaks:Hoses, intake boots and PCV parts are usually inexpensive; finding the leak is most of the job.
  • Mass airflow (MAF) sensor:Try an approved cleaning before replacing, and use the correct part for your engine; a wrong sensor can cause this code by itself.
  • Fuel pump:Replace the pump only after a pressure or volume test proves it weak; fuel-system work involves fire risk.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Saab

  • 2008 · 9-3

    Supported by vehicle data

    Saab's service information for the 2008 9-3 says the code sets when the long-term fuel adaptation passes about +17.5%, lists stalling, uneven running and misfire as possible symptoms, and tells the technician to check the crankcase ventilation system for leaks first.

Ford

  • 2021–2023 · F-150

    Supported by vehicle data

    On 2021-2023 F-150s, Ford links P0171 to the PCM software level; a software update is the bulletin repair.

  • TSB 25-2342Status unverified

    MIL on with P0171 from PCM software level

    2021–2023f-150 · issued · NHTSA 11020977

    Ford attributes a lit MIL with P0171 on listed F-150s to the PCM software level; the repair is a PCM software update. Supersedes 23-2195.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Chevrolet

  • 2012–2014 · Sonic

    Supported by vehicle data

    On 2012-2014 Sonics, a cracked air cleaner outlet duct after the MAF can set P0171; GM covers the duct replacement.

    NHTSA owner complaints mentioning P0171: 8. That shows what owners reported, not how often a cause sets the code.

  • 2020 · Silverado 1500

    Supported by vehicle data

    On some 2020 Silverado 1500s, a loose right-hand PCV tube causes an air-leak whistle and can set P0171 or P0174.

  • GCUS-9-3731Status unverified

    Cracked air cleaner outlet duct (P0171 or P0101)

    2012–2014sonic · issued · NHTSA 10126022

    A cracked air cleaner outlet duct can set P0171 or P0101; dealers replace the duct as needed under the special coverage.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • N192273400Status unverified

    Right-hand PCV tube not fully seated

    2020silverado-1500 · issued · NHTSA 10169635

    A loose right-hand crankcase ventilation (PCV) tube can cause a whistle or air-leak noise and, in the worst case, P0171 and/or P0174; dealers check that the tube is fully seated.

    We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

  • P0174Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

    Other bank Common general cause

    P0174 is the same lean test on the other bank. When both are stored, start with what both banks share: a large vacuum leak, the airflow sensor or fuel supply.

  • P0300Random or multiple-cylinder misfire detected

    Primary / secondary Common general cause

    Likely primary: P0171. A lean enough mixture can misfire, so random misfire codes may follow P0171. Fix the lean condition first, then recheck the misfire.

  • P0101Airflow sensor reading doesn't match what the engine computer expects for the throttle opening

    Shares causes Common general cause

    A skewed airflow sensor or an air leak after it can set both a MAF performance code and P0171.

  • P0102The airflow sensor's signal is lower than the engine computer will accept

    Shares causes Possible — needs testing

    A failed MAF circuit and P0171 can share a cause; fix the circuit code first because the computer cannot trim fuel properly without a good airflow signal.

  • P0420Catalytic converter efficiency below the computer's limit (Bank 1)

    Primary / secondary Possible — needs testing

    Likely primary: P0171. A sustained lean condition stresses the catalytic converter and can make its efficiency test fail. Fix P0171 before judging the converter.

Seen together

All Fuel trim (air-fuel mixture) codes

P0171 questions

Can I drive with P0171?

For short trips, if the engine runs normally, usually yes. Get it diagnosed soon: a lean engine can start to misfire, run hotter and damage the catalytic converter. If it stalls, hesitates badly or the light flashes, stop driving it.

Will a new oxygen sensor fix P0171?

Rarely. The sensor is usually reporting a real lean mixture. Replace it only if a test shows it responds slowly or reads falsely lean.

What is the difference between P0171 and P0174?

They are the same lean test on different sides of a V-type or flat engine. Bank 1 contains cylinder 1. When both appear together, look for a cause shared by both banks, such as a large vacuum leak, the airflow sensor or low fuel pressure.

Can a dirty air filter cause a lean code?

Unlikely on its own; a restricted filter reduces the air the sensor measures rather than adding unmeasured air. It is still worth checking the filter and the duct after it while you look for leaks.

Will my car pass an emissions inspection?

Not while the check engine light is on. After the repair, the light normally goes out after three consecutive drives without the fault, and the readiness monitors must complete before a retest.

Sources and review

  1. P0171 Code: Causes, Symptoms, and How to Fix It — Identifix ,
  2. Saab 9-3 (2008) WIS DTC P0171 00 (ME9): Long term fuel trim max value, air/fuel too lean — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  3. Bosch Oxygen Sensors consumer brochure — Robert Bosch LLC ,
  4. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II) for 2004 and later passenger cars, light-duty trucks and medium-duty vehicles — California Air Resources Board
  5. 2026 Honda Pilot Owner's Manual: Indicator Coming On/Blinking (Malfunction Indicator Lamp) — American Honda Motor Co.
  6. Saab 9-2X 2.5i (2005) WIS DTC P0172 System too rich (bank 1); P0171 uses the same procedure — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  7. Saab 9-3 (2008) WIS DTC P0101 00 (ME9): Mass air flow sensor performance (deviation from calculated mass air) — Saab Automobile (Saab WIS service information, hosted by Saab Club of North America)
  8. How to Clean a Mass Air Flow Sensor — CRC Industries
  9. P0420 Code: The Guide to Diagnosis and Fast Fixes — Identifix ,
  10. Ford Motor Company bulletin TSB 25-2342: MIL on with P0171 from PCM software level — Ford Motor Company (filed with NHTSA) ,
  11. General Motors bulletin GCUS-9-3731: Cracked air cleaner outlet duct (P0171 or P0101) — General Motors (filed with NHTSA) ,
  12. NHTSA ODI consumer complaints flat files — National Highway Traffic Safety Administration (NHTSA), Office of Defects Investigation
  13. General Motors bulletin N192273400: Right-hand PCV tube not fully seated — General Motors (filed with NHTSA) ,
  14. 40 CFR §85.2222 Onboard diagnostic test procedures — U.S. Environmental Protection Agency (Electronic Code of Federal Regulations)
Last reviewed
Written by
Saurabh K.
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