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DTC P0171: System Too Lean Bank 1: Notes

DTC detecting condition: 

NOTE: When a short period (distance) driving from the engine start to stop is repeated, engine warm-up followed may cause detection of P0172 due to gasoline evaporation within the engine oil and blow-by gas increase.

Trouble symptom: 

NOTE: After the faulty parts are repaired or replaced, perform the final check in Basic Diagnostic Procedure. Ref. to ENGINE (DIAGNOSTICS) (H4DO)>BASIC DIAGNOSTIC PROCEDURE>PROCEDURE .
  1. CHECK DTC  .
    1. Turn the ignition switch to ON.
    2. Using the Subaru Select Monitor or a general scan tool, read the DTC of [Engine]. Ref. to COMMON (DIAGNOSTICS)>DIAGNOSTIC TROUBLE CODE (DTC) .

    Is DTC other than P0171, P0172, P1170, or P117B displayed? (Current code)

    Yes:  Check the appropriate DTC using the "Diagnostic Trouble Code (DTC)". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>DIAGNOSTIC TROUBLE CODE (DTC) .

    No:  Go to  2.

  2. CHECK EXHAUST SYSTEM  .

    Check exhaust system parts.

    NOTE: Check the following items.
    • Looseness and improper fitting of exhaust system parts
    • Damage (crack, hole etc.) of parts
    • Loose or improperly attached front oxygen (A/F) sensor and rear oxygen sensor

    Is there any fault in exhaust system?

    Yes:  Repair or replace faulty parts.

    No:  Go to  3.

  3. CHECK AIR INTAKE SYSTEM  .

    Check the installing condition of the air intake system.

    Are there holes, loose bolts or disconnection of hose on air intake system?

    Yes:  Repair the air intake system. Ref. to INTAKE (INDUCTION) (H4DO)>GENERAL DESCRIPTION>COMPONENT .

    No:  Go to  4.

  4. CHECK FUEL PRESSURE  .
    WARNING: Place "NO OPEN FLAMES" signs near the working area.
    CAUTION:
    • Be careful not to spill fuel.
    • Before attaching/detaching a fuel pressure gauge, release the fuel pressure.
    1. Start the engine and warm up completely.
    2. Check the fuel pressure. Ref. to MECHANICAL (H4DO)>FUEL PRESSURE>INSPECTION .

    Is the check result OK?

    Yes:  Go to  5.

    No:  Check the fuel pump and fuel delivery line.

    Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FUEL PUMP ASSEMBLY>INSPECTION .

    Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FUEL DELIVERY AND EVAPORATION LINES>INSPECTION .

  5. CHECK ENGINE COOLANT TEMPERATURE SENSOR  .
    1. Start the engine and warm up completely.
    2. Using the Subaru Select Monitor, read the value in [Coolant Temp.].
    NOTE: For detailed operation procedures, refer to "COMMON (DIAGNOSTICS)". Ref. to COMMON (DIAGNOSTICS)>DATA MONITOR .

    Is the value of [Coolant Temp.] 80°C (176°F) or more?

    Yes:  Go to  6.

    No:  Replace the engine coolant temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>ENGINE COOLANT TEMPERATURE SENSOR .

  6. CHECK MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR  .
    1. Start the engine and warm up until the engine coolant temperature exceeds 75°C (167°F).
    2. For AT models, set the select lever to "P" range or "N" range, and for MT models, place the shift lever in the neutral position.
    3. Turn the A/C switch to OFF
    4. Turn all the accessory switches to OFF
    5. Using the Subaru Select Monitor, read the value in [Mass Air Flow].
    NOTE: For detailed operation procedures, refer to "COMMON (DIAGNOSTICS)". Ref. to COMMON (DIAGNOSTICS)>DATA MONITOR .

    Is the value of [Mass Air Flow] 2 - 5 [g/s] (0.26 - 0.66 lb/min)?

    Yes:  Go to  7.

    No:  Replace the mass air flow and intake air temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR .

  7. CHECK MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR  .
    1. Start the engine and warm up until the engine coolant temperature exceeds 80°C (176°F).
    2. For AT models, set the select lever to "P" range or "N" range, and for MT models, place the shift lever in the neutral position.
    3. Turn the A/C switch to OFF.
    4. Turn all the accessory switches to OFF.
    5. Open the front hood.
    6. Measure the ambient temperature.
    7. Using the Subaru Select Monitor, read the value in [Intake Air Temp.].
    NOTE: For detailed operation procedures, refer to "COMMON (DIAGNOSTICS)". Ref. to COMMON (DIAGNOSTICS)>DATA MONITOR .

    Subtract ambient temperature from [Intake Air Temp.]. Is the obtained value -10 - 50°C (-18 - 90°F)?

    Yes:  Go to  8.

    No:  Replace the mass air flow and intake air temperature sensor. Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>MASS AIR FLOW AND INTAKE AIR TEMPERATURE SENSOR .

  8. CHECK PCV HOSE  .

    Check PCV hose No. 1. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>PCV HOSE>INSPECTION .

    Is the check result OK?

    Yes:  Go to  9.

    No:  Repair or replace PCV hose No. 1. Ref. to EMISSION CONTROL (AUX. EMISSION CONTROL DEVICES) (H4DO)>PCV HOSE .

  9. CHECK FOR POOR CONTACT  .

    Check for poor contact of ECM connector.

    Is there poor contact of ECM connector?

    Yes:  Repair the poor contact of ECM connector.

    No:  Check the following items, and when either one matches, replace the engine oil and engine oil filter. Ref. to LUBRICATION (H4DO)>ENGINE OIL . Ref. to LUBRICATION (H4DO)>OIL FILTER .

    • It is assumed that the user tends to repeat short period (distance) driving from the engine start to stop, and also tends to drive before full engine warmed-up.
    • Compared to other engines of the same type, there is a lot stronger gasoline odor from the engine oil filler neck.

      Go to  10.

  10. CHECK DATA MONITOR  .
    1. Using the Subaru Select Monitor or a general scan tool, perform the clear memory of [Engine]. Ref. to COMMON (DIAGNOSTICS)>CLEAR MEMORY .
    2. Start the engine and warm up completely.
    3. Perform a no-load idling operation for at least 10 minutes.
    4. Using the Subaru Select Monitor, read the values in [Short term fuel trim B1] and [Long term fuel trim B1].
    NOTE: For detailed operation procedures, refer to "COMMON (DIAGNOSTICS)". Ref. to COMMON (DIAGNOSTICS)>DATA MONITOR .

    Is the sum value of [Short term fuel trim B1] and [Long term fuel trim B1] within ±20%?

    Yes:  Finish the diagnosis.

    No:  Go to  11.

  11. PERFORM ACTIVE TEST  .
    1. Start the engine.
    2. Using the Subaru Select Monitor, select [Injection Mode Change] of [Active Test], and select [Port Injection].
      NOTE: For detailed operation procedures, refer to the "Active Test". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>ACTIVE TEST .
    3. During [Active Test] in progress, read the values of [Short term fuel trim B1], [Long term fuel trim B1], [A/F Sensor #1], and [Target Equivalence Ratio].
    4. Using the Subaru Select Monitor, select [Injection Mode Change] of [Active Test], and select [Direct Injection].
      NOTE: For detailed operation procedures, refer to the "Active Test". Ref. to ENGINE (DIAGNOSTICS) (H4DO)>ACTIVE TEST .
    5. During [Active Test] in progress, read the values of [Short term fuel trim B1], [Long term fuel trim B1], [A/F Sensor #1], and [Target Equivalence Ratio].

    Are the diagnostic values of [Port Injection] and [Direct Injection] {[Short term fuel trim B1] + [Long term fuel trim B1] + ([A/F Sensor #1] - [Target Equivalence Ratio]) x 100} within ±15% respectively?

    [Port Injection] [Direct Injection] Reference
    OK (within ±15%) OK (within ±15%) A
    OK (within ±15%) NG (exceeding ±15%) B
    NG (exceeding ±15%) OK (within ±15%) C
    NG (exceeding ±15%) NG (exceeding ±15%) D

    A  : Finish the diagnosis.

    B  : Replace the fuel injector (for cylinder direct injection). Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FUEL INJECTOR .

    C  : Replace the fuel injector (for port injection). Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FUEL INJECTOR .

    D  : Replace the fuel injectors (for cylinder direct injection and for port injection). Ref. to FUEL INJECTION (FUEL SYSTEMS) (H4DO)>FUEL INJECTOR .

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