DTC P0138: Rear Secondary Heated Oxygen Sensor Circuit High - Honda Accord 2012-2017
DIY Honda Garage
[{'start': '5.000', 'end': '10.116', 'text': "The diagnostic trouble code (DTC) P0138 refers to an issue with the rear secondary heated oxygen sensor, specifically for bank 1, sensor 2 in a Honda Accord manufactured between 2012 and 2017. This particular sensor plays a crucial role in assessing the oxygen content in the exhaust gases emitted by the engine. Its primary function is to help manage the air/fuel mixture, ensuring optimal engine performance and emissions control. Located downstream of the rear warm-up three-way catalytic converter, the sensor provides feedback to the powertrain control module (PCM). When the sensor detects a high output, exceeding 1.27 volts continuously for at least five seconds, the PCM recognizes this anomaly as a malfunction. This will trigger the storage of the DTC P0138 to alert technicians that further investigation is required. DTC P0138 is primarily concerned with the rear secondary heated oxygen sensor, also known as secondary HO2S for bank 1, sensor 2. This sensor is strategically placed downstream of the rear warm-up three-way catalytic converter within the exhaust system. Its primary function is to measure the residual oxygen content in exhaust gases after they have passed through the catalytic converter. This information is vital for the powertrain control module (PCM) to adjust the air/fuel ratio for optimum combustion. If the sensor output is consistently high, indicating an overly rich exhaust mixture, the PCM detects this as a fault, which ultimately triggers the DTC P 0138. Understanding the role and position of this sensor helps in identifying issues that may arise during diagnostics and repairs. Several possible causes may lead to the triggering of DTC P0138, and it's essential to consider each of these for an accurate diagnosis. The first potential issue is a circuit failure associated with the rear secondary heated oxygen sensor. This could involve damaged wiring or connectors that compromise the sensor's ability to function correctly. The second cause is that the sensor output may be rich and stuck, meaning that it is delivering a constant high voltage signal to the PCM. Both scenarios warrant careful examination of the sensor wiring, connections, and actual sensor function to confirm the underlying issue before proceeding with repairs. Symptoms associated with DTC P0138 can significantly impact vehicle performance and should be carefully monitored. One immediate indication is the illumination of the Malfunction Indicator Light (MIL) on the dashboard, which signals a problem with the vehicle’s emissions system. Additionally, a driver may notice decreased fuel efficiency as the engine management system struggles to maintain an optimal air/fuel ratio due to incorrect sensor readings. In some cases, poor engine performance may be observed, including rough idling or decreased acceleration. These symptoms can lead to not only a frustrating driving experience but may also result in increased emissions, contributing to environmental impact. It is essential to address these symptoms early to avoid more severe issues. The first troubleshooting step involves starting the engine and allowing it to idle until the radiator fan activates. This step is critical as it prepares the vehicle for further diagnostic tests. Once the fan operates, the engine must be allowed to idle for a minimum of five seconds. This idle period allows the vehicle's systems to stabilize and gather baseline data from the various sensors, including the secondary heated oxygen sensor. It ensures that the PCM accurately assesses the sensor's output under normal operating conditions. This foundational step is crucial in understanding whether the fault is transient or persistent. Ignoring this may lead to unclear diagnostic outcomes. The subsequent troubleshooting step requires driving the vehicle at a steady engine speed that falls between 1,500 to 3,000 revolutions per minute (RPM) for a minimum of five seconds. This is imperative as it enables the PCM to evaluate the sensor's performance under load conditions, which can differ significantly from idle. By maintaining a constant speed, any discrepancies in the sensor output can be more readily identified. It is important that the driving conditions comply with traffic regulations to ensure safety. If any freeze data has been recorded, it is best to drive under those conditions for proper validation. Monitoring the sensor's response during this operating condition will yield critical information to support the diagnosis. In cases where freeze data has been logged, it is recommended to replicate the driving conditions that caused the DTC P0138 to be stored. This step is essential for confirming whether the fault can be reproduced under the same circumstances. Freeze data provides valuable insight into engine parameters and conditions at the time of the fault, allowing for targeted diagnostics. Drivers should ensure that they still operate within the bounds of traffic regulations and ambient conditions, prioritizing safety throughout the troubleshooting process. Collecting this data aids in accurately pinpointing the failure's origin, making subsequent repairs more effective. Once repairs have been conducted, a final verification process is necessary to ensure the DTC P0138 has been cleared effectively. This involves using a diagnostic scan tool to clear the DTC from the PCM memory. Following this, a test drive of the vehicle is crucial to monitor for any re-emergence of the code. The vehicle should be driven under conditions similar to what led to the initial malfunction. If the DTC does not reappear after three consecutive trips with successful diagnostic runs, it is an indicator that the repair was successful and the issue has been resolved. This verification step is vital not just for ensuring the immediate effectiveness of the repair, but for future vehicle reliability. To prevent issues related to the DTC P0138, engaging in regular preventive maintenance is essential. Routine checks of the emissions system can help identify any potential faults before they escalate into more significant problems. This includes ensuring that the wiring harness and connectors associated with the rear secondary heated oxygen sensor are intact and free of corrosion or damages. Additionally, keeping the sensor clean will help to ensure accurate readings and optimal functionality. Regularly scheduled maintenance visits can create a maintenance record that aids in spotting patterns that could foreseeably lead to issues. Being proactive can save time and money while promoting vehicle longevity. In conclusion, understanding DTC P0138 is crucial for effective vehicle diagnostics and repairs. This code specifically points to issues concerning the rear secondary heated oxygen sensor and signifies the importance of this component in maintaining engine performance and reducing emissions. Following systematic troubleshooting steps allows for accurate identification of the issue, ensuring that potential causes are thoroughly investigated. Addressing symptoms promptly can prevent further complications, reinforcing the necessity of diligent vehicle upkeep. Finally, engaging in preventive maintenance is essential; it alleviates issues before they arise and fosters reliable vehicle operation in the long run."}]