Technical Description of OBD-II Trouble Code
The O2 Sensor Negative Current Control Circuit/Open is experiencing an issue in Bank 1 Sensor 1.
What does that mean?
This is a generic diagnostic trouble code (DTC) that is applicable to many OBD-II vehicles manufactured from 1996 onwards. This includes vehicles from Mazda, VW, Acura, Kia, Toyota, BMW, Peugeot, Lexus, Audi, and others. While the code is generic, the specific repair steps may vary depending on the year, make, model, and powertrain of the vehicle.
A stored code P2251 indicates that the powertrain control module (PCM) has detected a negative current discrepancy in the upstream oxygen (O2) sensor for engine bank number one. Bank one refers to the engine bank that contains the number one cylinder. Sensor 1 denotes the upstream (pre) sensor. The negative current control circuit is a ground circuit.
To monitor the exhaust oxygen content for each engine bank and catalytic converter efficiency, the PCM relies on data input from heated oxygen sensors (HO2S).
Oxygen sensors are constructed using a zirconium dioxide sensing element located in the center of a vented steel housing. Small platinum electrodes are soldered between the sensing element and wire leads in the oxygen sensor harness connector. The O2 sensor harness connector plugs into the controller area network (CAN), which connects the HO2S wiring harness to the PCM connector.
Each HO2S is threaded (or studded) into an exhaust pipe or manifold. It is positioned so that the sensing element is near the center of the pipe. Exhaust gases from the combustion chamber exit through the exhaust manifold and flow through the exhaust system, including catalytic converters. These gases flow over the oxygen sensors. The exhaust gases enter the oxygen sensor through specially designed vent holes in the steel housing and swirl around the sensing element. The air drawn through wire lead cavities in the sensor housing fills a tiny chamber in the middle of the sensor. The heated air in the tiny chamber causes the oxygen ions to produce energy, which is recognized by the PCM as voltage.
Variations in the number of O2 ions in ambient air and the number of oxygen molecules in the exhaust cause the heated oxygen ions inside the HO2S to rapidly and repeatedly bounce between platinum layers. These variations in HO2S output voltage indicate changes in exhaust oxygen concentration to the PCM.
Voltage signal outputs from the HO2S are lower when there is more oxygen present in the exhaust (lean condition) and higher when there is less oxygen present in the exhaust (rich condition). This portion of the HO2S uses low voltage (less than one-volt).
In a separate section of the sensor, the HO2S is preheated using battery voltage (12-volts). When the engine temperature is low, battery voltage heats up the HO2S, allowing it to begin monitoring exhaust oxygen content faster.
If the PCM detects a voltage that does not meet acceptable parameters, a P2251 code will be stored and a malfunction indicator lamp (MIL) may illuminate. In most cases, multiple ignition cycles with a failure are required for MIL illumination.
A typical O2 Oxygen Sensor:
What is the severity of this DTC?
A control circuit problem in the HO2S can result in a poorly running engine and various drivability issues. A code P2251 should be considered severe and addressed as soon as possible.
What are some of the symptoms of the code?
Symptoms of a P2251 trouble code may include:
What are some of the common causes of the code?
Possible causes for this code may include:
What are some P2251 troubleshooting steps?
In order to accurately diagnose a code P2251, you will need a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reliable source of vehicle information.
Connect the scanner to the vehicle diagnostic port and retrieve all stored codes and relevant freeze frame data. It is advisable to write down this information in case the code proves to be intermittent. Next, clear the codes and test drive the vehicle. At this point, one of two things will happen. Either the code P2251 will be reset or the PCM will enter readiness mode.
If the code is intermittent and the PCM enters readiness mode, it may be more challenging to diagnose. The conditions that caused the P2251 code to be stored may need to worsen before an accurate diagnosis can be made. If the code is reset, continue with the diagnosis.
Use your source of vehicle information to access connector face views, connector pinout charts, component locator charts, wiring diagrams, and diagnostic flow charts specific to the code and vehicle in question.
Perform a visual inspection of the HO2S-related wiring and connectors. Replace any wiring that has been cut, burned, or damaged.
Disconnect the HO2S in question and use the DVOM to check the resistance between the negative current control circuit and any voltage circuits. If there is continuity, suspect a defective HO2S.
If the code P2251 continues to be reset, start the engine and allow it to reach normal operating temperature while idling in neutral or park. Connect the scanner to the vehicle diagnostic port and observe the oxygen sensor input data on the data stream. Narrow down the scope of the data stream to include only relevant data for a faster response.
If the oxygen sensors are functioning normally, voltage on the pre-oxygen sensors will continuously cycle between 1 and 900 millivolts once the PCM enters closed loop operation. Post sensors will also cycle between 1 and 900 millivolts but will settle at a certain point and remain relatively stable compared to the pre-sensors. HO2S that fail to cycle adequately should be considered defective if the engine is in good working order.
If the HO2S displays battery voltage or no voltage on the scanner data stream, use the DVOM to obtain live data from the HO2S connector. If the output data remains the same, suspect an internal HO2S short that will require replacement of the HO2S.
