P0138: OBD-2 error code interpretation and solutions

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Vehicle engine and/or automatic transmission

Code: P0138 - O2 Sensor Circuit High Voltage Bank 1 Sensor 2

Your vehicle's ECM (Engine Control Module) registers the P0138 code when the O2 sensor for bank 1, sensor 2, fails the voltage check. According to specifications, the voltage should be below 1.2 volts for more than 10 seconds, indicating a lack of oxygen in the exhaust gases. In simple terms, the oxygen sensor is malfunctioning, or the fuel/oxygen mixture in your engine is out of balance. Either way, this issue needs to be addressed.

This OBD code is commonly found across many car manufacturers such as Toyota, BMW, Kia, Mercedes-Benz, Ford, Volkswagen, Skoda, Nissan, Renault, and others.

Causes of OBD P0138

There are three potential causes that could trigger this fault code. Here’s what to check first when troubleshooting the code:

The Engine Control Module (ECM) detects a voltage spike above 1.2 volts due to oxygen loss in the exhaust flow. This signals a "lean" fuel condition to the ECM, which attempts to compensate by adjusting the fuel/oxygen ratio.
The ECM registers a voltage spike and stores the code before triggering the check engine light. This could indicate a faulty sensor or an issue with the fuel/oxygen ratio.
The ECM compensates for the high voltage issue by using other engine oxygen sensors to adjust the fuel injector output and balance the fuel/oxygen ratio.

Symptoms of Error P0138

How can you tell that your Engine Control Module (ECM) is registering the possible P0138 code? Here are typical symptoms of this issue:

Your engine might run "lean" during the sensor test, causing misfires and hesitation during startup or acceleration.
The check engine light will come on, and you should always read the codes when this indicator lights up on your dashboard.
Your engine might experience air and fuel delivery issues depending on the root cause of the sensor’s voltage spike and the resulting "rich" fuel/oxygen ratio.

How to Diagnose DTC P0138?

Let's talk about how to pinpoint the cause of your P0138. Here’s a step-by-step guide for troubleshooting this code:

Start by scanning for fault codes and freezing frame data. Clear the codes and run the engine to check if this is just a one-time occurrence.
During the second scan, monitor the oxygen sensor data to check if the voltage spikes and drops faster on bank 1, sensor 2 compared to the other oxygen sensors.
Next, perform a mandatory wiring check. Ensure all wires, insulation, and connectors are intact and free from corrosion.
After checking the wiring, inspect the oxygen sensor itself for contamination by liquids or physical damage.
Check for leaks in your exhaust system. You don’t necessarily need a smoke test, but if you have the right equipment or a home kit, it’s worth checking.
Check the manufacturer’s maintenance recommendations for any additional diagnostic steps or known issues with the exhaust system on your vehicle. There may be a known issue and a quick fix, or an ECM software update that needs to be installed.

How to Avoid Common Mistakes with OBD P0138 Code?

There is a voltage comparison warning between bank 1, sensor 1, and sensor 2 when diagnosing P0138: both sensors should operate at nearly the same voltage, but sensor 2 should indicate a lower O2 level as the catalytic converter eliminates excess fuel and O2. If you don’t notice a difference in O2 readings, it might be caused by another underlying issue.

Many DIYers and even some technicians sometimes skip checking sensor 2 for contamination by oil or coolant due to engine leaks, which could lead to low O2 values even if the sensor is functioning properly. Another potential issue that’s easy to miss is a damaged catalytic converter. If it’s clogged with carbon or has an air leak, it can cause unstable sensor readings and trigger the P0138 error.

How Serious is the P0138 Error?

High output voltage from the O2 sensor, caused by catalytic converter failure, can result in high output readings from O2 sensors. This leads to costly repairs and serious fuel gas system issues, including failure of state emissions tests. Moreover, a faulty sensor can result in improper ECU control of the air-fuel ratio in the engine, which may lead to contamination of the catalytic converter or fouling of the spark plugs due to carbon build-up. This can result in breakdowns, poor fuel efficiency, and potential long-term engine damage.

How to Fix the P0138 Code?

Replacing the O2 sensor with a new one (quick and moderately expensive)
Repairing/replacing the wiring or connectors to the O2 sensor for bank 1, sensor 2 (quick and cheap)
Replacing the catalytic converter before the sensor (costly repair)
Fixing injector leaks (potentially expensive fix)

Conclusion

Like many other issues in fuel and exhaust systems, this situation can go from a quick fix to significant repair expenses. When the check engine light comes on, it’s advisable to immediately read the fault codes using an OBD-II scanner or take the vehicle to a service center. Addressing the problem early in the detection phase can effectively save time, money, and prevent further complications.

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OBD P0138 code analysis and solutions

The page presents a detailed analysis of code P0138 for Opel, Renault, VW (Volkswagen), Ford, Peugeot, Mitsubishi, Skoda, BMW, Citroën, Toyota, Lada, Honda, Kia, and other models to investigate the possible causes and solutions related to obd-ii errors, with focus on the issue. We offer technical documentation and approaches for diagnostics to identify and find a solution effectively. Our resource is designed to be a comprehensive source of information on OBD 2 errors, providing you with the necessary tools for successful repair and maintenance of your vehicle. If during the repair of OBD-II codes you need to purchase parts, don’t forget to visit Avtopro!