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    P0171 System Too Lean (Bank 1)

    What does this OBD II code mean? 

    This generic OBD2 code is set when the Powertrain Control Module (PCM) senses an inappropriate combustion produced by a lean air/fuel mixture.
    This particular code is pointing out the bank 1, in other words, the bank of cylinders containing cylinder #1.

    Why does this OBD II code appear?

    Under closed loop operation, the PCM adjusts the amount of fuel delivered to the engine with the goal of reaching a perfect combustion and lower emissions to a minimum. Oxygen sensors play a key role during this process providing feedback regarding the O2 content present in the exhaust gases. Based on the information collected from its sensors, the PCM can determine if the engine is running rich or lean and then make the necessary corrections. 
    The OBD2 code P0171 is stored in memory when the PCM detects a lean mixture during a given period of time even after adjusting the volume of fuel delivered.

    What symptoms may occur with this OBD II code?

    Expect the typical symptoms from a lean mixture condition:

    • CEL lit. The Check Engine Light will turn on after few driving cycles to alert the driver about the situation. 
    • Possible misfires and/or spark knock. 
    • Poor fuel economy.
    • You may have rough idling.
    • Hesitation during acceleration, especially under wide open throttle (WOT) conditions.


    What are the causes that trigger this OBD II code?

    There're many factors that could cause a lean mixture condition including among others the following:

    • Damaged, burnt, corroded, open or shorted sensor wiring. 
    • Worn, corroded, burnt, disconnected or bent wiring harness connector pins. 
    • Air leaks in the intake manifold, PCV hose, vacuum hoses, vacuum driven components or MAF sensor tubing.
    • Low fuel system pressure caused by the fuel pump, clogged in-line fuel filter, obstructed or deficient fuel injectors, faulty fuel pressure regulator, or fuel lines leak.
    • Faulty, deficient or dirty mass air flow sensor (MAF)
    • Faulty or deficient upstream oxygen sensor.
    • Faulty or deficient PCV valve
    • Leak between the exhaust manifold and the upstream O2 sensor.
    • Even when unlikely, a defective PCM cannot be discarded.

    Please notice that possible causes are based on the assumption that only trouble code P0171 is stored in memory.

    Diagnostic strategy for troubleshooting this OBD II code

    Warning: If any other code directly related to the MAF sensor is present then you must start diagnosing and solving those codes first. 

    It's highly recommended starting the diagnostic process with the usual information gathering:

    • With the help of a professional OBD2 scan tool, preferably one with OEM capabilities, take note of all data trouble codes in memory, including pending codes.
    • Take note of freeze frame information (if present).
    • Query the vehicle owner about what, when and where the failure occurs. Does it happen with the engine hot? Cold? Under load? All the standard questions.
    • Take note of the vehicle VIN and find a reliable source of technical reference.

    Once you are done, the next step is discarding an intermittent condition. 

    • Using your scan tool clear DTC memory. 
    • Perform a complete driving cycle according to OEM procedures.

    If the code P0171 disappears then inform the owner about the intermittent nature of the code and explain that further testing with the code present will be required. If the code does appear then continue to the testing stage, consisting in discarding the possible causes of the problem. 

     

    IMPORTANT: the diagnostic procedure explained in this section should be taken as a guideline. Always refer to specific OEM procedures when possible.

     

    The workflow for the diagnostic process is:

    • The tests should be executed in sequential order, in other words, complete step 1 before going to step 2 and so on.
    • Repair, fix, or replace any component if you suspect is not in good condition.
    • If any repair, fix, reprogramming or component replacement is made during any step then you should clear DTC memory and repeat the whole KOER driving cycle.
    • If the DTC appear again then you should continue to the next step.
    • If the DTC does not appear again after completing the KOER tests then you can assume the problem is fixed, in that case, continuing with the remaining steps would be optional.

     

    The diagnostic process for the code P0171 is as follows:

    1. Sensor harness wiring and connector: carry out an exhaustive visual inspection of MAF sensor and oxygen sensor wiring and connectors. Pay special attention to burnt, corroded or deteriorated wiring and/or connector. Unplug each sensor and check for corrosion, damage, bent pins, or loosely connections. 
    2. Electrical tests: with the help of a digital volt-ohm meter (DVOM) and the adequate wiring diagram perform a KOEO check on the MAF and O2 sensors. Verify supply and/or reference voltage, electrical resistance, continuity, current and ground integrity. Compare the obtained values with the appropriate OEM literature.
    3. Vacuum and/or air leaks: perform a manual vacuum reading in the intake manifold test port or in any direct vacuum hose with no direct relation to the fuel injection system. If the vacuum reading is not within the normal value then perform a comprehensive visual inspection of all vacuum hoses including but not limited to PCV hose, fuel pressure regulator hose, MAP hose, brakes hose, etc, also look for intake manifold leaks and pay special attention to possible air or vacuum leaks downstream the MAF sensor. Also, check PCV and replace if defective. 
    4. Fuel system delivery: carry out a manual fuel pressure reading using a proper fuel pressure gauge and adapters. This is crucial, even if the scanner is reporting the fuel pressure. If the fuel pressure is below its normal value then perform an extensive fuel system test starting with the fuel pump followed by the fuel pressure regulator, inline fuel filter, fuel injectors performance, and fuel lines (leaks) inspection. 
    5. Exhaust system leak: any leak between the manifold and the upstream oxygen sensor will cause an incorrect reading of the exhaust gases. Inspect the bank #1 exhaust and repair any leak you may find. 
    6. Mass Air Flow sensor (MAF) maintenance: remove the sensor from the vehicle and perform a visual inspection of it. If you notice oil deposits or clogged conduits then a routine maintenance will be necessary. Using extreme caution clean the MAF sensor internal conduits. Avoid using any aggressive detergent (most commercial engine decarbonizing products can damage the sensor), preferably use biodegradable liquid soap with abundant water and Q-tips type cotton swabs. Let the sensor dry and install it on the vehicle. Repair any possible leak in its housing as necessary.
    7. Mass Air Flow sensor (MAF) diagnosis: proceed with a complete MAF sensor diagnostic using your scan tool, DVOM, and/or oscilloscope as required by the OEM literature. Replace MAF sensor depending on results. Once the sensor is discarded as a possible cause for this code continue with the next step.
    8. Upstream oxygen sensor: depending on vehicle year and manufacturer you may find installed a traditional oxygen sensor or a modern A/F sensor. They are very similar, however, the diagnostic procedure is not quite the same. Some manufacturers include built-in tests that help in the process, others not. Refer to the proper literature in order to perform a complete sensor diagnostic. Replace if needed. 
    9. Problems with the powertrain control module: the most unlikely cause for this code is a faulty PCM or a corrupted program. Yet, if you are completely certain that all prior tests passed then you must remove the PCM and check it, reprogram it or replace it as necessary.
       

     Do you need further help....?

    Edited by DamasoSanoja





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