The P0138 Trouble Code and How To Deal With It

Car diagnostic trouble code (DTC) regularly develop during the lifespan of a vehicle. The powertrain control module (PCM), otherwise referred to as the engine control module (ECM), detects and stores the DTC codes within its main memory and illuminate the check engine light. For instance, the o2 sensor circuit high voltage trouble code stems from an inconsistent supply of the air to fuel ratio inside the engine. This airfuel difference ratio is detected by the O2 Sensor Circuit, which sends distress signals to the PCM. 

In most cases, the owner will notice a lightened service engine soon warning light as well as a derailed fuel economy. It is also common for the vehicle to display instances of a rough idle when it develops the P0138 code. But what is the P0138 code?

What does P0138 mean?

This standard OBD-II trouble code indicates that there is a discrepancy in the airfuel ratio intake in the engine. Low air supply and too much rich fuel into the engine or an excessive air intake along with too lean fuel are standard instigators of the P0138 trouble code. Usually, when the PCM registers this trouble code, the dashboard check engine light turns on. The O2 sensor circuit is responsible for examining the exhaust gas and detecting the air to fuel ratio difference. Some common symptoms that signify the engine code P0138 include:

• Rough idle

• Poor fuel economy

• “On” check engine light

• Bad gas mileage

• Lagging / missing engine

• Emissions test is a fail


Here are additional signs, apart from the OBD-II scanner, that helps in diagnosing the P0138 code: Strong emissions, the engine regularly runs lean and the engine ultimately dies. But what causes the P0138 diagnostic code?  

Causes of the P0138 code

There are numerous sources for the check engine P0138 trouble code, many of which stem from the heated oxygen sensor. Among common causes of the error code is the oxygen sensor circuit high voltage. Here the oxygen sensor circuit fails to regulate the power output below 12 volts for 10 seconds, which indicates low oxygen levels. Here are other factors that might initialize the P0138 trouble code:

Corrosion of wires

When wires connecting the PCM to the O2 sensor corrode, signal transmission is hindered, leading to inefficient fuel consumption. In effect, the P0138 error code is initiated, prompting the PCM to readjust fuel and air consumption. Preventive maintenance conducted towards the heated sensors and its wiring could reveal such defects in time, which then would require a mechanic to replace. It is recommendable that you purchase genuine wire harness to act as replacements of the old ones. One way to prevent wire corrosion is to insulate the wires using durable material. 

Elevated fuel pressure

If fuel pressure suddenly elevates, it might interrupt the fuel to air ratio intake in the engine. Fuel pressure, when heightened, leads to the formation of too lean fuel. At the exhaust, the change will be detected by the O2 sensor, which then sends appropriate signals to the powertrain control unit. As a result, the PCM adjusts and transmits similar signals to correct the functioning of the individual components. The use of an OBD-II scanner should help reveal the trouble code, which is then cleared off the PCM’s memory.

Flagship one LOGO p0172 code

Dysfunctional O2 sensor circuit

A dysfunctional O2 sensor ultimately impairs the vehicle’s ability to balance air intake and fuel consumption in the engine. Since this component primarily regulates fuel consumption, it would be effortless to notice that it is defective. First, the amount you will be spending on fuel will increase gradually over time instead of suddenly hiking upwards. Second, you will notice that the check engine light is illuminated. Third, the P0138 diagnostic trouble code will initialize immediately. 

The wiring is faulty   

Faulty wiring between the car’s PCM and the sensor signal circuit is among major causes of the P0138 trouble code. Disconnection in the wiring harness is also another wiring fault that is responsible for the above error code. It is possible that the faulty wiring is as a result of exposure to everyday driving conditions. Age is also another factor to consider as it causes the wires to wear out, ultimately disconnecting. However, this should be simple to resolve as it would only require a skilled mechanic to configure the wiring correctly.

Fixing P0138 trouble code

If the P0138 trouble code develops in your vehicle, the root cause is mostly the heated O2 sensor, which might be defective. Sometimes a damaged PCM or a high voltage O2 might be the cause of trouble code instead. It is recommendable to find replacements of the O2 sensor and PCM if they are completely damaged. This should be a dauntless task, especially in sophisticated regions, where reputable spare parts suppliers are in abundance. Suppliers like FLAGSHIP ONE., INC provide old vehicle models with fabricated PCMs, transmission control units (TCM) as well as body control modules (BCM) that have lifelong warranties. The provision of free shipping services throughout the U.S is also available during purchases. 


The DTC P0138 results due to a variety of reasons not limited to the O2 sensor circuits alone. It might also arise due to a defective fuse box, leaking fuel injector, blocked catalytic converter, electrical connection issues and a possible short, among others. Therefore, you should take the car to a skilled mechanic who will then use a scanning tool to root the code. Afterward, the mechanic will erase the error code and then take the car for a test drive to ascertain whether or not everything is working as before.

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