P0174 Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

System too lean, bank 2

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

P0174 in short

P0174 means the engine computer is adding as much extra fuel as it is allowed on bank 2 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. P0174 sets on bank 2 (the side without cylinder 1, so only V-type and flat engines) 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 2 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 2

    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 2 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 2 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 2 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

Nissan

  • 2017–2019 · Armada

    Supported by vehicle data

    On 2017-2019 Armadas with only P0174 stored, Nissan's first step is an ECM software check and reprogram.

  • NTB20-009Status unverified

    MIL on with only P0174 stored

    2017–2019armada · issued · NHTSA 10172250

    When P0174 is the only code, Nissan has the technician confirm the ECM part number and reprogram the ECM if an update applies.

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

Ram

  • 2021 · 1500

    Supported by vehicle data

    On some 2021 Ram 1500s, a blocked fuel/EVAP line bundle can set P0174 with weak acceleration or rough starting.

  • 1400521Status unverified

    Fuel line blockage with P0174

    20211500 · issued · NHTSA 10221675

    A blocked engine fuel/EVAP forward line bundle can set P0174 with reduced acceleration or rough starting; the bundle is replaced.

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

Chevrolet

  • 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 P0174 or P0171.

  • 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.

  • P0171Engine computer has reached its limit adding fuel on bank 1: the mixture is lean

    Other bank Common general cause

    P0171 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: P0174. A lean enough mixture can misfire, so random misfire codes may follow P0174. 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 P0174.

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

    Shares causes Possible — needs testing

    A failed MAF circuit and P0174 can share a cause; fix the circuit code first because the computer cannot trim fuel properly without a good airflow signal.

  • P0430Catalytic converter efficiency below the computer's limit (Bank 2)

    Primary / secondary Possible — needs testing

    Likely primary: P0174. A sustained lean condition stresses the catalytic converter and can make its efficiency test fail. Fix P0174 before judging the converter.

Seen together

All Fuel trim (air-fuel mixture) codes

P0174 questions

Can I drive with P0174?

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 P0174?

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 P0174 and P0171?

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. Nissan North America bulletin NTB20-009: MIL on with only P0174 stored — Nissan North America (filed with NHTSA) ,
  11. FCA US (Stellantis) bulletin 1400521: Fuel line blockage with P0174 — FCA US (Stellantis) (filed with NHTSA) ,
  12. General Motors bulletin N192273400: Right-hand PCV tube not fully seated — General Motors (filed with NHTSA) ,
  13. 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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