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

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.

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.

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.

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.

–Richard McCuistian, ASE Certified Master Automobile Technician

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.

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.

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:

• Illuminated check engine light
• Engine misfire
• Excessive fuel consumption
• Rough idling
• Lack of acceleration
• Stalling

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.

Where to Get a Quality Oxygen Sensor to Fix the P0138 Code

A bad oxygen sensor interferes with the engine and exhaust system, causing issues like reduced fuel efficiency. Ignoring the P0138 code might lead to more serious problems later on. Fortunately, you can order a new oxygen sensor at a competitive price from CarParts.com.

Make CarParts.com your one-stop shop for oxygen sensors and other replacement automotive parts. We exclusively source our products from trusted manufacturers, ensuring that you get the performance and reliability you deserve. Our website is easy to navigate. You can quickly find the right part that fits your vehicle by entering specific details into the integrated vehicle selector. Enjoy fast shipping that can deliver your new parts to your doorstep in as fast as two business days as long as you order by 12 PM ET.

So what are you waiting for? Check out our extensive selection of oxygen sensors and find one that fits your vehicle and budget!

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