System too lean, bank 1 (P0171)

A confirmed cause of the lean condition, demonstrated by fuel trims returning within the normal band at idle and under load.

Record freeze-frame data and the long- and short-term fuel trims at idle, at 2500 rpm and under load before changing anything. Note whether the fault affects one bank only.

• petrol Not verified
  This vehicle applicability has not been confirmed against the manufacturer documentation for your exact vehicle.
  Applies to gasoline engines with closed-loop fuel control. Direct-injection systems additionally require the manufacturer high-pressure procedure.

1. 1

   Vacuum or intake air leak after the airflow meter

   high
2. 2

   Contaminated or failing mass airflow sensor

   high
3. 3

   Low fuel pressure or a restricted filter

   high
4. 4

   Dirty or restricted fuel injectors

   medium
5. 5

   Leaking intake manifold gasket

   medium
6. 6

   Exhaust leak upstream of the oxygen sensor, or a lazy sensor

   medium
7. 7

   Leaking crankcase ventilation or brake booster hose

   medium

1. 1

   Diagnostic

   Read the fuel trims in three conditions

   Record long- and short-term fuel trims at idle, at 2500 rpm with no load, and under load with the vehicle driven or on a dynamometer. Compare bank 1 with bank 2 where the engine has two banks.

   What did you find? A clear pattern: lean at idle only, lean across the range, or lean under load only.

   Measurement: %

   Authored outcomes

   • Caution Lean at idle and correct at speed
     Classic pattern for unmetered air: a vacuum or intake leak matters most when airflow is low.
   • Caution Lean across the whole range
     Suspect measured airflow or fuel supply rather than a small leak.
   • Fault Lean under load only
     Fuel delivery under demand: pump, filter or pressure regulation.
2. 2

   Diagnostic

   Check airflow measurement and fuel supply

   Compare the indicated airflow at key-on and at idle with what the engine displacement and speed imply. Inspect the sensor element for contamination and the intake duct for splits after the sensor. Measure fuel pressure and, where possible, flow under load.

   What did you find? A plausible airflow reading and fuel pressure within the general range for the system.

   Authored outcomes

   • Fault Airflow reading is implausible or the element is contaminated
     Clean or replace the sensor, then recheck the trims. Confirm with live data rather than assuming.
   • Fault Fuel pressure or flow is below the general range
     Diagnose the fuel delivery side: filter, pump, pressure regulator.
   • Normal Airflow and fuel supply are normal
     Look for unmetered air.
3. 3

   Diagnostic

   Hunt for unmetered air

   With the engine idling, introduce a controlled, non-flammable leak-detection medium around intake joints, the throttle body gasket, the manifold gasket, the brake booster hose and the crankcase ventilation hoses. Watch the fuel trims rather than listening for a change in engine note alone.

   What did you find? No change in trims when a joint is treated, and no visible damage on hoses or gaskets.

   Authored outcomes

   • Fault Trims move when a joint is treated
     That joint leaks. Repair it and recheck the trims at idle and at speed.
   • Inconclusive No leak found
     Proceed to the exhaust and oxygen sensor check.
4. 4

   Diagnostic

   Check the exhaust and the oxygen sensor

   Inspect for exhaust leaks upstream of the sensor. Compare the upstream oxygen sensor signal with the commanded mixture and with the second bank where available. Look for a sensor that is slow to switch or biased low.

   What did you find? No exhaust leak, and an upstream sensor that switches normally.

   Authored outcomes

   • Fault An exhaust leak or a biased sensor is found
     Repair the leak or replace the sensor, then confirm the trims recover.
   • Inconclusive No fault found here either
     Reconfirm the trims after any repair already carried out. If they are still lean, the fault is likely a small, cumulative combination of small leaks and injector restriction.

1. 1

   Verification

   Confirm the repair

   Clear the code, road test the vehicle through the conditions recorded at the start, and re-read the fuel trims at idle and under load.

   What did you find? Trims within the normal band in both conditions, and no returning code.

   Authored outcomes

   • Normal Trims recovered and no code returns
     The repair is confirmed.
     End of this branch
   • Fault The code returns
     The cause was not found. Return to the trim pattern and revisit the branch that matches it.
     End of this branch

• Long-term fuel trim within the normal band at idle
• Long-term fuel trim within the normal band at 2500 rpm
• No pending or stored P0171 after a road test
• Live airflow reading plausible for engine speed and load

• Fuel trims reveal hidden problems General educational guidance · AutoTools
• Fault code P0171 — system too lean, bank 1 Trusted technical reference · AutoTools OBD reference
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