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Is your vehicle triggering a P0138 trouble code on your OBD-II scanner? If so, read on to find out what this code means, what the symptoms and possible causes are, and how you can diagnose and fix the issue.

What Does the P0138 Code Mean?

Diagnostic trouble code (DTC) P0138 stands for O2 Sensor Circuit High Voltage (Bank 1, Sensor 2). The code will set when your car’s primary computer—also called the powertrain control module (PCM)—detects that, for a given period of time, the voltage signal from the rear O2 sensor is too high.

oxygen sensor in an exhaust system
Code P0138 will set when your car’s primary computer—also called the powertrain control module (PCM)—detects that, for a given period of time, the voltage signal from the rear O2 sensor is too high.

Honda describes the P0138 this way:

The secondary heated oxygen sensor (HO2S) (sensor 2) detects the oxygen content in the exhaust gas downstream of the three way catalytic converter (TWC) during stoichiometric air/fuel ratio feedback control based on the air/fuel ratio (A/F) sensor (sensor 1) output voltage. The secondary HO2S controls the air/fuel ratio from the A/F sensor output voltage to optimize TWC efficiency.

After current is applied to the secondary HO2S heater, if the secondary HO2S output continues to exceed the upper limit used during feedback control, a malfunction is detected and a DTC is stored.

For a detailed technical discussion of how the downstream oxygen sensor can trigger P0138, read the next section.

If you want to learn the possible causes of this code, click here.

Heated oxygen sensor in the exhaust of a car focused
If the secondary HO2S output continues to exceed the upper limit used during feedback control, a malfunction is detected and a DTC is stored.

How the Downstream Oxygen Sensor Can Trigger P0138

All modern cars have at least two oxygen sensors. Although both sensors detect the amount of oxygen in the exhaust stream, the PCM uses the data they provide differently. The first sensor (or Sensor 1) is the upstream oxygen sensor; that sensor is the primary feedback input in the “closed loop” fuel system, and aids the PCM in regulating the vehicle’s air/fuel ratio.

Okay, so let’s talk a bit more about sensor 2, which is also referred to as the downstream oxygen sensor. This sensor’s primary job is to monitor the catalytic converter’s oxygen storage capacity.

What many mechanics don’t know is that, on most vehicles, if the upstream (primary) O2 sensor signal becomes unreliable to the point that the ECM/PCM knows it can no longer trust that signal, the downstream O2 sensor that usually monitors oxygen storage capacity in the catalytic converter will become the ECM/PCM’s primary input for fuel trim adjustments. Why? Because the algorithms written into the ECM/PCM give priority to protecting the catalytic converter, which is the most expensive and important emissions component in the system.

A traditional zirconia upstream oxygen sensor produces a voltage signal that continuously switches back and forth. The reading gives the PCM an idea of the engine’s air/fuel mixture. A value above 0.45 volts indicates a rich condition, while a reading below 0.45 volts indicates a lean condition. The upstream sensor should be very actively switching while the system is in “closed loop,” which means it’s constantly using the upstream sensor for fuel control.

downstream oxygen sensor on catalyst pipe of old gasoline car
If the catalytic converter and the upstream sensor are working properly, the downstream oxygen sensor should drift just above .5 volts in a lazy fashion.

Meanwhile, if the catalytic converter and the upstream sensor are working properly, the downstream oxygen sensor should drift just above .5 volts in a lazy fashion. It should never mirror the operation of the front sensor if the catalyst is properly storing oxygen.

Code P0138 is set when the downstream sensor sends too high of a voltage signal for an extended period of time. Too high typically means higher than 1.1 volts, which is very high for a traditional zirconia style sensor. The bank one portion of the code indicates that the issue is on the side of the engine that houses the number one cylinder (only applies to engines with a ‘V’ or boxer configuration). If you’re working on an engine with two banks and four sensors, make sure you know which is Bank 1 and which is Bank 2 on the vehicle you’re working on so you’ll know you’re working on the right O2 sensor.

Bank 1 is where the #1 spark plug is. Always. Again, determine which bank is which first or you’ll waste a lot of time and effort. O2 sensor 1/1 is the upstream sensor on Bank 1 (different for a Ford than for a Chevy or a Dodge), and O2 sensor 1/2 is the downstream sensor on Bank 1. O2 2/1 is upstream on Bank 2 and O2 2/2 is downstream on Bank 2.

Since the code is for sensor 2, we know that we’re dealing with the downstream oxygen sensor. 

What are the Possible Causes of the P0138 Code?

Although the P0138 code often appears due to a worn-out oxygen sensor, it’s not the only condition that can trigger this error.

This diagnostic code can also appear when any of the related components fail.

That’s why, apart from knowing the symptoms to watch out for, it is also a good idea to know what causes a P0138 error code. Below is a list of common causes:

  • Faulty oxygen sensor
  • A rich running condition caused by a fuel delivery issue (e.g., excessive fuel pressure)
  • Damaged wiring or connections
  • A problem with the PCM, such as software in need of an update

What are the Common Symptoms of the P0138 Code?

With a P0138 code, here are the common symptoms you should keep an eye out for:

exhaust pipe of a car
Engine misfire and excessive fuel consumption are two common symptoms of Code P0138.

If your vehicle exhibits any of the signs above, get your vehicle diagnosed properly by a certified mechanic.

How to Diagnose the P0138 Code

Getting the right diagnosis for any error code is extremely difficult. However, we live in the information age, so guides and troubleshooting material for the error code P0138 are easily accessible.

Below are some videos you can look at to get a better grasp of the diagnosis process for a P0138 code:

How to Fix the P00138 Code

There are individual solutions for each root cause of certain trouble codes—there’s no one way to fix or address a specific error code. The same goes for a P0138.

Vehicles can also be vastly different from one another. There may be different ways to address a particular error code, depending on the year, make, and model of your vehicle—so always check your owner’s manual before attempting a fix.

Repair manuals, such as those from Chilton, are useful. However, you may want to get an ALLDATA subscription if you’re serious about DIY auto repair. They have single-vehicle subscriptions for DIYers that provide detailed factory repair information.

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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.

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Comments

  1. Tracy says

    i have a 2018 z71 colorado v6 check engine light came on i hooked scanner up and it told o2 senser circut high voltage were should i start

    • Hi Tracy,

      Code P0138 can be set for a variety of reasons, ranging from a faulty O2 sensor to a rich running condition. Are there any other DTCs stored in addition to that code? Any driveability problems?

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