OBD-II Trouble Codes

P0138 Code: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

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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.

To get a better grasp of the meaning of this code, you must first understand what an oxygen sensor is.

oxygen sensor in an exhaust system
Code P0138 means that your powertrain control module (PCM) has detected that the voltage signal from the rear O2 sensor is too high.

What is an Oxygen Sensor?

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 aids the PCM in regulating the vehicle’s air/fuel ratio.

Sensor 2, also known as the downstream oxygen sensor, monitors the catalytic converter to check its performance.

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.

Meanwhile, if the catalytic converter is working properly, the downstream oxygen sensor should produce a fairly continuous reading of around 0.45 volts.

Code P0138 is set when the downstream sensor sends too high of a voltage signal for an extended period of time. 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).

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:

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

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:

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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