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

Did your scan tool show the P0138 code? Do you wonder what it means and whether you can continue to drive?

Keep reading to have a first evaluation of the situation. 

  • P0138 Definition: O2 sensor circuit high voltage (Bank 1, Sensor 2)
  • Code Type: Generic – P0138 indicates the same problem whether you’re having a Jeep, Toyota, or Dodge etc.
  • Can I drive with the P0138 code? Yes, but keep driving will cause internal engine damage.
  • Easy to fix? Intermediate level.
  • Cost: $20 – $95 (common)

Luckily, P0138 is safe to drive in a short period!

But to avoid more harm to the engine, let’s read on to know the causes and how to fix these problems!

Table Of Contents

What Does The P0138 Code Mean?

The P0138 is triggered when the downstream O2 sensor (bank 1) voltage is too high (>1.2 volts) for more than 10 seconds. This high voltage indicates that the oxygen level in the exhaust gas is abnormally low. 

What Is The O2 Downstream Sensor’s Function?

Downstream oxygen sensors (located after the catalytic converter) monitor the conditions of the catalytic converter.

The less air in the exhaust gas (after passing the cats), the higher the downstream O2 sensor’s output voltage. 

Because the catalytic converter already processed the exhaust gas, the oxygen level is stable. Therefore, a normal working downstream O2 sensor stays around 0.45 volts.

What Can Trigger The P0138 Code? 

The sensor voltage being too high can be explained by two scenarios:  

  • The reading is incorrect – results from the sensor’s wiring harness or the sensor itself
  • The reading is correct – results from rich conditions

I’ll explain the details in the below parts.

But first, let’s navigate your cause. 


P0138 Causes Identification: Quick View

P0138 can appear alone or with other related codes. This table will help you navigate the root causes as well as the solutions for them. 

Codes CombinationCausesSolutions
Only P0138Short in the O2 sensor signal circuit

Faulty downstream O2 sensor
Find and replace the damaged wire

Replace the sensor
P0138 + P0172/LTFT1<-10%Too much fuel in the air-fuel mixtureThere are several causes leading to this issue, further investigation is needed. 

Note: The causes for each code combination are the most common ones. There can be some uncommon issues hidden under those codes.

P0138: Causes, Symptoms, and How to Fix

Now, you may have a general idea of what causes the P0138 error code. 

Read on to learn how to pinpoint the root problem and stop being a part-changer!

Cause #1: Short In The O2 Sensor Signal Circuit

This is the first thing you need to check. 

Short to battery voltage in the O2 sensor signal circuit will make the sensor’s output voltage increase, triggering P0138. 

If the wiring is the problem, there is no symptom other than the Check Engine Light. This is because downstream O2 sensors are not used to adjust the air-fuel ratio (like upstream.) 

And there will be no effect on the engine performance. 

Cause #2: Faulty Downstream O2 Sensor

After making sure all the wiring is good, a faulty downstream sensor is very likely to be the cause of P0138.

Again, there will be no symptoms in this case. 

How to fix it? 

All you have to do is to replace the sensor. 

Bank 1 sensor 2 is located right after the catalytic converter. The replacement process is straightforward and the part itself is inexpensive. So, you can totally do it by yourself. 

Tips: You will need a hammer and a wrench to break the sensor loose. 

You can watch this video from 1A Auto and replace the downstream oxygen sensor yourself to save money.

Cause #3: Rich Condition

A rich condition indicates that there is more than enough fuel in the air-fuel mixture, 

This leads to a very low level of oxygen level in the exhaust gas, causing the computer to set P0138. 

Symptoms of rich conditions causing P0138: 

  • Poor fuel economy
  • Smell of gasoline
  • Slow acceleration
  • Negative long-term fuel trim on bank 1 (LTFT1<-10%)
  • Combination of P0138 and P0172

There are many different causes that can lead to this condition. 

How to know which problem your car is having? 

First, you need to monitor the long-term fuel trim on both bank 1 (LTFT1) and bank 2 (LTFT2.) 

  • If Both LTFT1 and LTFT2 < -10%, the fuel pressure is too high (which is usually caused by bad FPR, fuel pump, fuel rail, etc.)
  • If LTFT<-10% and -5%<LTFT2<5%, a sensor (on bank 1) is causing rich mixture (upstream O2 sensor, MAF, MAP, IAT, etc.) 

After all, rich conditions causing P0138 are not something you should worry about. Most of the time, replacing an inexpensive sensor will fix the problem. The hard part is to know which one needs to be replaced. 

For further diagnosis, I wrote an article about the code P0172. Read it and you can exactly pinpoint your problem!  

How Much Does It Cost To Fix The Code P0138?

9 out of 10 times, you can just spend less than 100 bucks to fix the issues related to the bank 1 downstream O2 sensor. 

If you get negative long-term fuel trim (rich conditions,) there can be many different solutions depending on the causes. 

The Estimated Repair Cost Of P0138

Repair O2 sensor circuitDIY: $10 - $20
Mechanic: $130 - $140
Replace (downstream or upstream) O2 sensorDIY: $20 - $95
Mechanic: $100 - $450
Replace MAF/MAP sensorDIY: $30 - $200
Mechanic: $80 - $280
Replace the IAT sensorDIY: $20-$150
Mechanic: $40-$250
Repair IAT sensor wiringDIY: $0-$20
Mechanic: $20-$75
Replace the fuel pressure regulator (FPR)DIY: $50 - $200
Mechanic: $150 - $350
Replace the fuel pumpDIY: $50 - $850
Mechanic: $220 - $1,100

Note: The data in this table was collected in July 2022. The actual price depends on many factors, such as your car’s make and year, mechanic’s rate, etc.

You Ask, I Answer

Through this article, you will find it easier to identify the causes and the corresponding solutions to avoid more unnecessary expenses.

So, did you find out the reason causing P0138 on your car? Please leave a comment below.

And if you successfully erase this code by another way, don’t hesitate to share us your method. I am looking forward to know your ideas.

See ya!

Read more: The 9 Best OBD2 Scanners for 2022: The Only Review You Need

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