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    P2251 O2 Sensor Negative Current Control Circuit Open Bank 1 Sensor 1

    What does this OBD II code mean? 

    This generic OBD2 code is set when the Powertrain Control Module (PCM) senses an open ground circuit in the upstream oxygen sensor.
    This particular code is pointing out the bank 1, in other words, the bank of cylinders containing cylinder #1. The sensor 1 tag is indicating the upstream O2 sensor located just before the catalytic converter.

    Why does this OBD II code appear?

    Most of the oxygen sensors use a 4-wire circuit, but 3-wire circuits and in some cases 5+ wire circuits are also used. What all of them have in common is the “heater circuit”. The heater element helps the sensor to reach its operating temperature faster. The PCM is constantly monitoring the heater circuit, specifically its voltage and/or current. Depending on the year and vehicle maker, the heater circuit can be commanded by the supply voltage (positively controlled) or by the ground circuit (negatively controlled). In both cases, the powertrain control module (PCM) is in charge of activating the circuit when considered necessary. Because of that, the PCM can compare in real time the heater circuit voltage (or current depending on design) with a reference value that it's no other than the vehicle operating voltage. Usually, a discrepancy greater than 10% is enough to alert the PCM about a potential short or open circuit. 
    The OBD2 code P2251 is stored in memory when the PCM detects an open circuit in the ground line used for control. Depending on the sensor design the ground line may be shared between the heater circuit and the sensor data circuit.

    What symptoms may occur with this OBD II code?

    Expect the typical symptoms from a P2251 code:

    • CEL lit. The Check Engine Light will turn on after few driving cycles to alert the driver about the situation. 
    • Poor fuel economy
    • Possible misfires, spark knocking or detonations. 
    • Possible rough idle and/or engine hesitation. 
    • Sometimes you may expect a lack of power, especially during hard acceleration.
    • In general, expect any condition directly related to an incorrect air-fuel mixture.

    What are the causes that trigger this OBD II code?

    There're many factors that could cause a code P2251 including among others the following:

    • Damaged, burnt, corroded, open or shorted upstream O2 sensor wiring. 
    • Worn, corroded, burnt, disconnected or bent upstream O2 wiring harness connector pins. 
    • Faulty or deficient upstream oxygen sensor.
    • Even when unlikely, a defective PCM cannot be discarded.

    Diagnostic strategy for troubleshooting this OBD II code

    Warning: If any other code directly related with the upstream oxygen 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 P2251 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 P2251 is as follows:

    1. Sensor harness wiring and connector: carry out an exhaustive visual inspection of bank 1 upstream oxygen sensor wiring and connectors. Pay special attention to burnt, corroded, open, shorted to ground or deteriorated wiring and/or connector. Unplug the sensor and check for corrosion, damage, bent pins, etc.
    2. Electrical tests: with the help of a digital volt-ohm meter (DVOM) and the adequate wiring diagram perform a KOEO and KOER check on the heater circuit for the bank 1 upstream O2 sensor. Verify supply and/or reference voltage, electrical resistance, continuity, current and ground integrity. Compare the obtained values with the appropriate OEM literature. It’s crucial during this stage to perform the adequate tests depending on circuitry design (positively controlled or negatively controlled). 
    3. Upstream oxygen sensor: some manufacturers include built-in tests that help in the process of diagnosing the O2 sensor, others not. Refer to the proper literature in order to perform a complete sensor diagnostic. Replace if needed. As usual, after any repair clear the DTC memory, repeat KOER driving cycle and check if the code persists. If the code appears again then continue with the next step.
    4. 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.
       

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