P0171 and P0174 together: both banks running lean

Monitor

If the engine idles and drives normally, two lean codes are an emissions and efficiency fault you can drive on briefly while booking a diagnosis, but a lean condition on both banks can worsen into misfire and converter damage.

It gets more serious if:

  • Inspect soon: Inspect soon if the engine idles roughly, hesitates or stalls, or if any misfire code (P0300 to P0306) is stored with them: a mixture lean enough to misfire puts the catalytic converter at risk.
  • Stop: Stop if the check engine light starts flashing: that is misfire bad enough to damage the catalytic converter. Pull over where nothing flammable is under the car and let it cool.

Source: how we rate severity

The two codes

  • P0171: Engine computer has reached its limit adding fuel on bank 1: the mixture is lean
  • P0174: Engine computer has reached its limit adding fuel on bank 2: the mixture is lean

Why they show up together

P0171 and P0174 are the same fuel-trim test run on each side of a V6 or V8. Each one means the computer has added as much fuel as it is allowed to on that bank and the mixture still reads lean.

When both banks hit the limit at the same time, it makes sense to check first what both banks share: the airflow sensor that measures air for the whole engine, the fuel supply that feeds every injector, or an air leak in the common part of the intake. A one-bank fault such as a single injector or one side's intake gasket would normally upset only one bank.

That is diagnostic reasoning rather than a rule: two separate one-bank faults are possible, so whole-engine checks come first but do not rule them out.

Shared causes

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

  1. Vacuum leak in the shared intake (hoses, PCV system, intake boot after the airflow sensor)

    Common general cause
    Why it ranks here
    Air that enters after the airflow sensor is never measured, so the computer under-fuels every cylinder it feeds. MOTOR lists vacuum leaks first among lean causes and describes a both-bank lean case traced to the PCV system.
    What would change this
    Raised if fuel trims are high at idle and drop toward normal at 2500 rpm, or if a smoke test shows a leak. Lowered if trims stay high at higher engine speed.
    How to test the vacuum hoses and intake leaks

  2. Airflow (MAF) sensor reading less air than is really entering

    Common general cause
    Why it ranks here
    The airflow reading sets the base fuel amount for the whole engine. A contaminated sensor that under-reports airflow makes both banks lean, and MOTOR discusses the MAF among items to check when both banks fail fuel trim.
    What would change this
    Raised if the airflow reading is low for the engine speed and load, or if trims improve after cleaning the sensor with a dedicated cleaner. Lowered if the reading matches the expected value.
    How to test the mass airflow (maf) sensor

  3. Low fuel pressure or volume (weak pump, restricted filter or supply)

    Common general cause
    Why it ranks here
    Every injector draws from the same supply, so a supply shortfall leans both banks together. MOTOR lists low fuel pressure or volume as a lean cause and points to the fuel pumps when both banks fail fuel trim.
    What would change this
    Raised if trims get worse under load or a fuel pressure and volume test comes in low. Lowered if pressure holds to specification under load.
    How to test the fuel pump

Diagnose in this order

  1. Read freeze-frame data and look for any other stored codes first, especially airflow sensor codes (P0101, P0102) and misfire codes (P0300 to P0306). A more specific code is the better lead.
  2. With a scan tool, compare fuel trims on both banks at idle and again at a steady 2500 rpm. Trims that are high at idle but come down at higher speed point to an air leak; trims that stay high or get worse with load point to fuel supply or an airflow sensor that under-reads.
  3. Check the intake for unmetered air: cracked or disconnected vacuum hoses, the PCV system, and the intake boot after the airflow sensor. A smoke test finds leaks that cannot be seen or heard.
  4. Inspect the airflow sensor for dirt or oil contamination and compare its reading with the expected value for the engine. Clean it only with a cleaner made for airflow sensors.
  5. If no air leak or airflow fault is found, have fuel pressure and volume tested under load, because a weak pump or restricted supply starves both banks together.

Sources and review

  1. Lean On Me — MOTOR Magazine ,
  2. Fuel Trim Data: A Powerful Diagnostic Tool — MOTOR Magazine ,
  3. MOTOR Shop Talk: Troubleshooting Lean Fuel Conditions (Roy Dennis Ripple) — MOTOR Magazine ,
  4. Ford P0171 & P0174 Lean Codes (Larry Carley) — AA1Car.com
  5. Random Misfires (OBD II P0300 DTC) — AA1Car.com
  6. Trouble Shooter: 2002 Trooper, Hit or Miss — MOTOR Magazine ,
  7. Protect your MAF sensor with a clean air filter — Delphi Technologies
  8. How to Clean a Mass Air Flow Sensor (MAF): A Complete Guide — CRC Industries
  9. Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II), 2004 and subsequent model-year passenger cars, light-duty trucks and medium-duty vehicles (2015 amendments) — California Air Resources Board
  10. 2023 Integra Owner's Manual: Indicator, Coming On/Blinking — American Honda Motor Co.
  11. OBD consumer brochure: Do you drive a 1996 or newer car or light truck? (EPA420-F-02-016) — U.S. Environmental Protection Agency, Office of Transportation and Air Quality ,
Last reviewed
Written by
Saurabh K.
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