You can find information about the P0138 fault code on this page.
It is one of the most popular fault codes that people search for on this website (in the top 50).
We have given it a seriousness of 6/10 with 1 being the least serious and 10 being the worst.
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Fault code P0138 is logged when the engine control unit (ECU) has detected a high voltage from an oxygen sensor. It is specific to oxygen sensor 2 in bank 1 of the engine.
This error occurs when the output signal from the oxygen sensor is greater than 1.2 V for a period of 10 seconds or more. This is indicative of a lack of oxygen in the exhaust.
Bank 1 refers to the side of the engine in which cylinder 1 is located. This can be found in the owner’s manual. Oxygen sensor 2 is the downstream sensor.
A faulty sensor can cause the vehicle to run improperly as the ECU is being fed incorrect information. In this instance, the information helps determine the fuel injection process.
Other Signs of This Problem
The various signs of fault code P0138 are related to the running of the engine as the ECU is being given too little or incorrect information from the sensor.
The Check Engine light is one of the most obvious signs.
Other issues can be how the car runs. It is possible the vehicle will have a rough idle.
Additionally, the engine will run rich. This can lead to black smoke from the exhaust. It can also cause a decrease in fuel economy and performance.
What Could Be The Problem?
Most of the potential causes are entirely related to the electrical components and circuitry.
One of the most likely issues is a faulty downstream oxygen sensor. This will obviously provide an incorrect reading that can signal the fault code to be logged.
Additionally, there could be an issue with the wiring or connections. A frayed or damaged wire can disrupt the signal being sent.
This can also happen as a result of an open or short circuit in the wiring.
Other system errors like excessive fuel pressure can also lead to this issue.
All of the fixes for P0138 are focused on fixing any existing electrical issues.
It is a good idea to reset the codes and perform a test drive to determine if the code is logged again.
If the oxygen sensor is in fact faulty, it should be replaced. The fuse running to bank 1 sensor 2 should also be checked and replaced if need be.
The wiring and connections between the sensor and the ECU should also be checked for damage or fraying. These should be fixed accordingly.
It is also important to ensure that the connections are properly fixed to the components.
Fault code P0138 should be considered a moderately serious problem. There is no immediate danger to the engine or its components but operating in this state can eventually cause damage.
It can also cause the vehicle to run very poorly after enough carbon buildup is caused. The fuel economy will also be very poor.
It should ideally be fixed as soon as possible.
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