P0138: Diagnostic Fault Code Popular Triggers and Solutions

P0138 is a trouble code you may come across when your car’s Engine Control Module (ECM) detects prolonged high voltage from the downstream oxygen sensor. When this happens, this fault code shows a lack of enough oxygen in the exhaust gases.

P0138 ~ Ran When Parked

In this post, you will learn the most popular triggers and solutions to this diagnostic code.

Why Is Your Car Triggering the P0138 Diagnostic Code?

The reason your car is triggering the P0138 diagnostic code is because of a faulty downstream oxygen sensor. If your car’s oxygen sensor is defective, it can trigger this diagnostic code by constantly sending a high-voltage signal to the ECM. As a result, it will trigger the check engine light.

Once the downstream oxygen sensor is faulty, it cannot accurately measure the amount of oxygen in the exhaust gases. This can cause erratic voltage readings. One possible outcome is a consistently higher voltage because of inaccurate sensor measurements. When the ECM detects this it interprets it as an issue with the catalytic converter.

A faulty sensor could arise from contamination, damage, or wear and tear. Often, the oxygen sensor lasts up to 100,000 miles. If the sensor lasts beyond this, it can be less accurate. Also, with time, damage from heat or impact can damage the sensor.

In addition, contamination from gasoline, carbon deposit, or oil can result in O2 sensor circuit high voltage signals to the ECM, which triggers this error code. Virtually all vehicles can trigger this diagnostic code. Here are some that will trigger this fault code:

  • P0138 Toyota
  • P0138 Jeep
  • P0138 Honda
  • P0138 Chevy Malibu
  • P0138 Nissan

Wiring or Connector Issues With Your Car’s O2 Sensor

If the wiring harness associated with your car’s oxygen (O2) sensor has issues, it can trigger this diagnostic code. For example, if there is a short circuit in the wiring, it can trigger this fault code by sending a high voltage signal to your car’s ECM. The short circuit creates an unintended pathway that can cause abnormal behavior.

Also, if your car has a wiring or connector issue with the O2 sensor it can trigger this fault code. This will happen if the wiring issue results in a high voltage that is inconsistent with the amount of oxygen in the exhaust. You will likely face this issue if the O2 sensor wiring is damaged, loose, corroded, or insulation failure.

Why Is Car Triggering the P0138 Diagnostic Code ~ Ran When Parked

Over time, your car’s O2 sensor wiring can be damaged by heat, physical impact, or chemicals. This can break the wiring causing the O2 sensor to send signals indicating the oxygen levels are low than expected. The same can happen when the connectors are loose, corroded, or the insulation has failed.

In addition, O2 wiring or connector issues can interfere with the O2 sensor signal or power supply. As a result, it will affect the sensor accuracy. Eventually, your car’s ECM will throw p0138 bank 1 sensor 2 location fault code indicating there is low oxygen in the exhaust gases than intended.

Dirty or Defective Catalytic Converter Can Trigger P0138

Another reason your car will trigger the P0138 fault code is because of a dirty or malfunctioning catalytic converter. This will mostly happen if your car’s catalytic converter is clogged, damaged, or contaminated.

The most common contaminants are oil, additives, coolant, or fuel. A malfunctioning catalytic converter decreases the efficiency of the exhaust emission control system.

As a result, it will allow more pollutants to pass through. Consequently, this will be reflected in the O2 sensor’s readings. It will show that there is not enough oxygen in the exhaust gases triggering the P0138 fault code. The reason is the clogged catalytic converter is unable to effectively remove pollutants in the exhaust gases.

This will cause the O2 sensor to detect lower-than-normal oxygen content in the exhaust gases, triggering this fault code.

Using Incompatible O2 Sensor Can Trigger P0138 Fault Code

If you recently replaced your car’s O2 sensor, the explanation for this fault code could be using an incompatible O2 sensor. Different sensors have different calibrations. For instance, different O2 sensors cater to different upstream and downstream applications.

A good example is using upstream O2 sensors that usually measure the air-fuel mixture before entering the combustion chamber.

Furthermore, different O2 sensors are calibrated to work in specific voltage ranges and response times. These characteristics correspond to the emission control systems for which the sensors are designed. Thus, if you use a sensor with incorrect calibration or characteristics it can trigger this fault code since it does not reflect the emission status.

As a result, when you use the wrong O2 sensor, its output voltage may not align with the expected values of a downstream O2 sensor. As a result, the O2 sensor can show readings that are higher than normal. When your ECM detects this, it will trigger the error in question.

Damaged O2 Sensor Terminal Can Trigger This Diagnostics Code

Another explanation for the P0138 diagnostic code in your car is a damaged O2 sensor terminal. When your car has a damaged O2 sensor, it can trigger this fault code because it interferes with the correct transmission of signals between the sensor and the ECM. The damage can be as a result of poor connection, corrosion, or physical impact.

A damaged O2 sensor can affect the flow of current and even interfere with the accurate transmission of voltage signals. This will result in inconsistent voltage signal transmission resulting in fluctuating readings.

Since the ECM continuously monitors the O2 sensor, if the downstream sensor voltage readings are consistently high it will trigger this diagnostic code.

A Rich Air-Fuel Mixture Can Trigger P0138 Fault Code

If your vehicle has a rich air-fuel mixture, it can trigger this fault code since it affects the readings of the downstream O2 sensor. When that is the case, the voltage of the downstream O2 sensor will remain high for a long time which suggests an issue with emissions control. This issue may arise because of a faulty fuel injector or faulty sensor readings.

When your car has a rich air-fuel mixture, it results in a lot of unburned fuel in the exhaust gases. This happens because the excess fuel consumes some of the oxygen during combustion.

Eventually, the downstream O2 sensor will measure the remaining oxygen level in the exhaust gases after going through the catalytic converter. Even after going through the catalytic converter, the oxygen can be relatively low than expected.

The downstream O2 sensor can read lower oxygen levels than expected because of the excess fuel. This will result in consistently high readings of voltage from the sensor indicating low levels of oxygen.

When the O2 sensor consistently reports higher voltage levels, the ECM may interpret this as a sign of a faulty catalytic converter. This can trigger the fault code in question to suggest an issue with emissions control.

Battery Issues Can Trigger This Diagnostic Fault Code

When your car has battery issues, it affects the ability of the ECM to operate effectively. The correct voltage plays a role in accurate sensor readings. However, with time, your car’s battery terminals and cables can accumulate dirt, corrosion, and debris which result in poor connections.

Battery Issues Can Trigger This Diagnostic Fault Code ~ Ran When Parked

Poor connections and drained batteries cause voltage fluctuations. The fluctuations in voltage levels can affect the downstream O2 sensor’s voltage readings. This can result in erratic or skewed sensor readings that can make the ECM trigger this fault code.

What Is the Best Solution to P0138 Diagnostic Code?

The best solution to P0138 diagnostic code involves replacing the faulty downstream oxygen sensor. When you replace the faulty O2 sensor with a new and properly working sensor, you restore the correct transmission of signals. The new O2 sensor provides ECM with the correct voltage stopping the P0138 fault code.

With the new O2 sensor, your car’s ECM gets precise data on the oxygen levels in the exhaust gases. Provided the converter is functioning properly, the downstream O2 sensor voltage will stabilize to the expected range.

Best Answer to P0138 Diagnostic Code ~ Ran When Parked

As a result, it resolves this fault code, and your car’s emissions control returns to the expected function. At the same time, it will clear the check engine light that this fault code triggered.

When replacing the faulty O2 sensor, make sure you use one that is compatible with your car’s specifications. Also, keep in mind that to replace the downstream O2 sensor, you will need tools such as a socket set, a jack and jack stands as well as an anti-seize compound. The O2 sensor repair cost varies between $200 and $300.

Repair/Replace Faulty O2 Sensor Wiring or Connector

You can also solve the P0138 diagnostic code by repairing or replacing the faulty oxygen sensor wiring or connector. By doing this, you restore accurate transmission of signals between the ECM and the O2 sensor. The repair or replacement involves fixing any breaks in the wiring including failed insulation.

This restores the proper connection and circuit integrity enabling accurate transmission of signals to the ECM. If your car’s catalytic converter is functioning, the voltage readings of the O2 sensor will be within range.

This will resolve this diagnostic code and restore the proper function of your car’s emissions control system. To replace or repair O2 sensor wiring, you will need the services of a professional mechanic.

Inspect and Clean Battery Terminals To Solve P0138

Inspecting and cleaning your car’s battery terminals and cables and cables can resolve this code by restoring the proper flow of current. This will help the O2 sensor function as it should and send accurate signals to the ECM. To clean the battery terminals and cables, you will need a few things such as a wrench, wire brush, water, and baking soda.

The process of cleaning is straightforward. First, you need to disconnect the cables starting with the negative battery cable. While at it, make sure you do not touch both terminals at the same time. Next, inspect the cables and terminals. If you notice corrosion, use the wire brush to remove the corrosion.

Clean the terminals with a damp cloth and dry them. You can then apply petroleum jelly to the terminals to avoid corrosion in the future. Once you are done, reconnect the cables starting with the positive battery cable and then the negative. Now, clear the trouble code. If that is the cause, you will solve the issue.

Replace or Repair Leaking Fuel Injector To Solve This Code

You can also replace or repair a leaking fuel injector to solve this fault code. This will address the cause of the rich air-fuel mixture. As a result, you will restore the balance between the air and fuel ratio decreasing the amount of unburned fuel in the exhaust. In return, it will normalize oxygen levels in the combustion chamber.

By correcting the air-fuel mixture, your car’s downstream O2 sensor readings will stabilize to the expected range. As a result, the ECM will detect the normal range and will stop triggering the fault code. Like most of the solutions to this fault code, you will need the services of a qualified mechanic.

P0138 Conclusion ~ Ran When Parked

Conclusion

In this post, you have learned the various triggers and solutions to the P0138 fault code. To recap, here is a concise summary of the key takeaways and how to fix P0138 code:

  • One of the most common triggers is a faulty oxygen sensor.
  • Other causes are O2 sensor wiring issues, catalytic converter issues, using the wrong O2 sensor, and a rich air-fuel mixture.
  • The best solution is to replace a faulty downstream oxygen sensor.
  • Also, you can solve this fault code by repairing or replacing faulty O2 wiring, addressing catalytic issues, using the correct O2 sensor, and optimizing the air-fuel mixture.
  • To avoid common mistakes when diagnosing and solving this fault code, you will need to contact a professional mechanic.

With this understanding, you can effectively avoid diagnosis mistakes when dealing with this fault code and solve it effectively.

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