Diagnostic Trouble Code P0138

P0138 “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)”


What is code P0138?

Code P0138 stands for “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)”. This diagnostic trouble code can show up for a number of problems related to the post-catalyst oxygen sensor. Therefore, it is important to have a qualified technician diagnose the specific problem that caused the code to be stored in your vehicle’s computer.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.

What does code P0138 mean?

A code P0138 suggests to a technician that there is a problem with the oxygen sensor that is mounted in the exhaust system after the catalytic converter. The ECM (Engine Control Module) has detected the problem and turned on the check engine light.

No matter how finely-tuned your engine might be, it will still emit harmful greenhouse gasses when it is running. These gasses are a natural byproduct of the combustion process. Because of this issue, the federal government has required for many years that auto manufacturers install a device called a catalytic converter in the exhaust system to mitigate those harmful pollutants before they can leave the tailpipe.

The catalytic converter (sometimes referred to as the “catalyst” due to the chemical reaction brought about by the components inside) contains a matrix filled with platinum or other precious metal, such as palladium or rhodium. These metallic chemical elements serve as the catalyst for various chemical reactions with the exhaust gasses that pass through. The result of these reactions is that harmful carbon monoxide, hydrocarbon, and nitrogen oxide are converted into carbon dioxide, water, and nitrogen and released into the atmosphere.

Because the function of the catalytic converter is so important, a pair of oxygen sensors (O2 sensors) are placed in the exhaust stream, one ahead of the catalytic converter and the other behind. When the ECM detects too much voltage from the downstream oxygen sensor, indicating a rich operating condition, it stores code P0138 and turns on the check engine light. Vehicles with “V” style engines (as in V6 or V8) might have more than one set of oxygen sensors. In those cases, this code relates to “Bank 1”, the side of the engine that contains the number 1 cylinder.


What causes code P0138?

A number of causes can trigger a code P0138. That is why an accurate diagnosis is essential. Causes of this code include:

Most likely

  • Bad oxygen sensor ($200-$500)
  • Faulty wiring to the oxygen sensor

Other possibilities


What symptoms are associated with this code?

Common signs associated with a code P0138 include the following:

  • The check engine light is on or flashing
  • Reduced fuel economy due to excess fuel consumption
  • Failed emissions test
  • Engine shakes, runs roughly, or lacks power
  • Engine stalling or difficult to start
  • Black smoke coming from the tailpipe


How does a technician diagnose code P0138?

If a diagnostic scan of your vehicle indicates a code P0138, a technician will perform tests to pinpoint the problem. These tests include checking for any related trouble codes. The technician will also use the scan tool to read the live data from the oxygen sensors and check for problems with the wiring that leads to the sensors. Electrical testing of the sensor using a multimeter might also be done. The technician might even remove the oxygen sensor from the vehicle for testing. Other tests might be conducted to determine if, instead, the problem is caused by faulty engine fuel management.


How serious is this code?

In some cases, a code P0138 will not cause immediate drivability issues. However, driving your vehicle with this code means that the ECM cannot accurately monitor the efficiency of the catalytic converter. It also means your engine could be running excessively rich. Neither of these is an imminent risk, but they can cost you thousands in damage if left unchecked for too long.

Get this checked out soon by a CarAdvise shop, where the work will be guaranteed, and one of our expert techs will review any repair orders.


Related Diagnostic Trouble Codes

P0132 “O2 Sensor Circuit High Voltage (Bank 1, Sensor 1)”

P0137 “O2 Sensor Circuit Low Voltage (Bank 1, Sensor 2)”

P0158 “O2 Sensor Circuit High Voltage (Bank 2, Sensor 2)”


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