P0138 Code – Meaning, Causes, Symptoms (& How To Fix)

Any time the engine doesn’t run as it should, the Check Engine Light can illuminate on your car’s dashboard. You might be able to see the P0138 Code after using your OBD-II scan tool.

You can get professional help to understand P0138 and determine the cause. Also, we look at symptoms that you may notice and offer some solutions. 

Definition of Code P0138

P0138 – O2 Oxygen Sensor Circuit High Voltage (Bank1, Sensor2)

What does the P0138 code mean?

P0138 DTC (generic trouble code) is an indication that the voltage at the oxygen sensor circuit in Bank 1, Sensor 2 is higher than normal. The Powertrain Control Module(PCM) determines that the voltage from the sensor exceeds what is shown in the manufacturer's specifications. 

The heated oxygen sensor can be found behind the catalytic convert. It should create an output signal that’s similar to the oxygen storage capacity of the catalytic converter. Bank 1 can be found at the end of the engine that has the number 1 cylinder. 

The downstream oxygen sensor's purpose is to measure the efficiency of the catalyst. Sometimes, you may also see the P0420 code along with P0138.

RELATED: Bank 1 vs Bank 2 – Sensor 1 & 2 (Locate O2 Sensors Fast & Easy)

P0138 Trouble code Symptoms

The car should continue to run as usual in most cases even though the P0138 code is set. Sometimes, you might just notice the Check Engine Light coming on. But, it is possible for the engine to perform less well.

These are some of the most common P0138 codes symptoms.

The P0138 code: Causes

The P0138 trouble code is most likely due to an oxygen sensor failure. However, this isn’t the only possible cause, which is why a complete diagnosis is required. 

Below are some top causes for the P0138 code.

What is the P0138 code?

Medium – Many people continue driving with the P0138 code, especially when it doesn’t exhibit any noticeable symptoms. However, it’s always best to have the problem looked at sooner than later.

If you run the engine without a proper mixture of fuel and air, it could lead to more damage. Instead of having a massive repair bill after neglecting the engine, it’s best to have it looked at right away. 

How can you fix the P0138 code?

After going through the detailed diagnostic steps below you'll have a better understanding of what to fix. Here are some of the more common methods to fix the P0138 DTC. 

• Replace downstream oxygen sensor
• Repair damaged connection/wiring
• Resolve fuel delivery problem 
• Replace/update PCM 

Common mistakes in diagnosing P0138

The most common mistake when dealing with the P0138 trouble code is to replace an oxygen sensor that isn’t bad. Because the DTC signifies that there’s an issue with the oxygen sensor, many people will replace it without doing a full diagnosis.

It could be a problem with wiring or connection, which may be less expensive to fix. There’s also the possibility that something is wrong with the fuel delivery, which is causing more fuel to enter the system than it should. That’s why you should follow the steps listed below before you replace any parts. 

How to diagnose P0138 Trouble code

You don't need any special tools to troubleshoot the P0138 DTC. Here are a few steps to consider following, as long as they don’t contradict what is found in your car’s service manual.

1. Pay attention to the trouble codes. You should also look for any other codes. 
2. Examine the wiring and connections. It is important to inspect all connections and wiring leading to and from oxygen sensor. It’s also wise to test the voltage of the battery power. 
3. An OBD scanner can check the voltage of the O2 sensor. If it is 0.9V or higher, it’s possible that the sensor is bad. You can consult your repair manual to determine the right values for O2 sensors on specific models of cars.

Any other information should be taken to a mechanic professional for diagnosis. 

Repair costs estimated at P0138

You don’t want to perform any fixes without running through all of the diagnostic steps first. Otherwise, you could spend money on parts that don’t need to be replaced. Once you know what’s wrong, you have a better idea of how to fix the problem. Here are some possible repair costs if you have a common problem. 

• Replace downstream oxygen sensor – $175-$450
• Repair damaged connection/wiring – $50-$550
• Resolve fuel delivery problem $125-$750
• Update/replace PCM – $225-$2,500

Here are some mechanics tips about the P0138 Code

It is located downstream of the catalytic convert. It’s responsible for measuring the air and fuel mixture that comes from the catalytic converter, making sure the cat is functioning as intended. 

A voltage reading that is higher than 0.9 Volts will indicate too much fuel. However, too little voltage, usually less than 0.1 volts will indicate that there is too much fuel in the mixture. 

If everything works as it should, oxygen sensor 2 will read an average voltage of 0.45 volts. The PCM will not alert if the voltage rises for longer than 10 seconds.