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AutoCodes – Generic – P0138

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

Quick Answer

The Heated Oxygen Sensors (HO2S) are used for fuel control and catalyst monitoring. Common causes include Faulty Rear Heated Oxygen Sensor Bank 1, Rear Heated Oxygen Sensor Bank 1 harness is open or shorted, Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection. Severity is Moderate - service should be scheduled soon. The vehicle may still be drivable for short trips if it runs normally, but avoid hard driving and repair it soon.

What it means
P0138 indicates O2 Sensor Circuit High Voltage Bank 1 Sensor 2.
Common causes
Common causes include Faulty Rear Heated Oxygen Sensor Bank 1, Rear Heated Oxygen Sensor Bank 1 harness is open or shorted, Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection.
Severity
Moderate - service should be scheduled soon.
Can I drive?
The vehicle may still be drivable for short trips if it runs normally, but avoid hard driving and repair it soon.
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Code Popularity 158,562
How popular is this code, among other codes? High
What is this?
Table of contents Created and updated by William Orellana on May 27, 2022
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Code P0138 Description

The Heated Oxygen Sensors (HO2S) are used for fuel control and catalyst monitoring. Each HO2S compares the oxygen content of the surrounding air with the oxygen content of the exhaust stream. When the engine is started, the Engine Control Module (ECM) operates in an Open Loop mode, ignoring the HO2S signal voltage while calculating the air-to-fuel ratio. While the engine runs, the HO2S heats up and begins to generate a voltage within a range of 0 - 1, 000 mV. Once sufficient HO2S voltage fluctuation is observed by the ECM, Closed Loop is entered. The ECM uses the HO2S voltage to determine the air-to-fuel ratio. An HO2S voltage that increases toward 1, 000 mV indicates a rich fuel mixture. An HO2S voltage that decreases toward 0 mV indicates a lean fuel mixture.
The heating elements inside each HO2S heat the sensor to bring the sensor up to operating conditions faster. This allows the system to enter Closed Loop earlier and the ECM to calculate the air-to-fuel ratio sooner.
The Diagnostic Trouble Code (DTC) will be set when the ECM detects that the HO2S voltage is greater than 1, 050 mV.
OBDII Code P0138 - O2 Sensor Circuit High Voltage Bank 1 Sensor 2 - AutoCodes.com
P0138 Code - O2 Sensor Circuit High Voltage Bank 1 Sensor 2
What does this mean?

What are the Possible Causes of the DTC P0138?

  • Faulty Rear Heated Oxygen Sensor Bank 1
  • Rear Heated Oxygen Sensor Bank 1 harness is open or shorted
  • Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection
  • Low or high fuel system pressure
  • Fuel contamination
  • Faulty fuel injectors
  • Exhaust gas leaks
  • Engine vacuum leaks

How to Fix the DTC P0138?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What does this mean?

Code Tech Notes

Replacing the Rear Heated Oxygen Sensor Bank 1 usually takes care of the problem.





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What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0138 code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

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Information for Specific Makes

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about P0138 Code

1. What are the common symptoms of OBDII code P0138?

- Common symptoms include poor fuel economy, rough idling, stalling, and an illuminated Check Engine light.

2. What does OBDII code P0138 indicate?

- OBDII code P0138 indicates that the O2 sensor circuit for Bank 1 Sensor 2 is reporting a high voltage condition.

3. What are the possible causes of OBDII code P0138?

- Possible causes include a faulty O2 sensor, wiring issues in the sensor circuit, exhaust leaks, or a problem with the engine's fuel system.

4. How can I diagnose the cause of OBDII code P0138?

- Diagnosing the cause typically involves using a scan tool to monitor the O2 sensor's voltage readings and inspecting the sensor and related wiring for damage.

5. Is it safe to continue driving with OBDII code P0138?

- It is not recommended to continue driving with OBDII code P0138 as it can lead to reduced fuel efficiency and potential damage to the catalytic converter.

6. Can I reset the Check Engine light after fixing OBDII code P0138?

- Yes, once the underlying issue causing the code is resolved, the Check Engine light can be reset using an OBDII scan tool.

7. How can I repair OBDII code P0138?

- Repair options may include replacing the Bank 1 Sensor 2 O2 sensor, repairing or replacing damaged wiring, fixing exhaust leaks, or addressing fuel system issues.

8. Will OBDII code P0138 cause my vehicle to fail an emissions test?

- Yes, OBDII code P0138 typically indicates a problem with the emissions control system and may cause a vehicle to fail an emissions test.

9. How do I prevent OBDII code P0138 from recurring?

- To prevent OBDII code P0138 from recurring, ensure regular maintenance of the O2 sensors and related components, and address any issues promptly.

10. Can a DIY enthusiast fix OBDII code P0138, or is professional help needed?

- Depending on the cause of the code and the individual's mechanical skills, a DIY enthusiast may be able to fix OBDII code P0138, but professional help may be needed for more complex issues.

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