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P0138 Code: Meaning, Symptoms, Causes, Diagnostics, and Fixes

The oxygen sensors in your engine help to maintain the proper air to fuel ratio. When they malfunction, it can prevent your engine from adjusting the air/fuel mix when it should. The P0138 code tells you the voltage of oxygen sensor 2 is reading too high for an extended period of time.

The P0138 trouble code points you to the exact area of your engine where the problem’s occurring. This makes it easier to diagnose than many codes. A full diagnosis is still necessary, however, to isolate whether the problem comes from the sensor itself, the wires around it, or somewhere else in the exhaust system.

If your vehicle is giving you a P0138 trouble code, you should fix it before it causes further engine damage. Read on below to learn more about what this code means and how to address it. 

When the voltage of oxygen sensor 2 is reading too high for too long, the code P0183 is triggered
The P0138 code tells you the voltage of oxygen sensor 2 is reading too high for a long time

P0138 Code Definition

P0138 Code Definition (Generic): O2 oxygen sensor circuit high voltage (bank 1, sensor 2)

P0138 Ford Code Definition: O2 sensor circuit high voltage bank 1 sensor 2

P0138 Jeep Code Definition: O2 sensor ½ circuit high

P0138 Nissan Code Definition: H02S12 circuit high voltage

P0138 Toyota Code Definition: Oxygen sensor circuit high voltage bank 1 sensor 2

What Does P0138 Mean?

The mix of air to fuel in your engine has to be maintained at 14.7:1 for the cylinders in the engine to fire correctly. There are a few components in your engine dedicated to ensuring this mix is maintained correctly. Key among them are the catalytic converter and the oxygen sensors.

There are two oxygen sensors in your system. The upstream oxygen sensor is labeled as sensor 1, and its input is used by the powertrain control model (PCM) to adjust the mix to be leaner (more air) or richer (more fuel). The downstream oxygen sensor is labeled as sensor 2. It measures the vapors coming out of the catalytic converter to verify it’s functioning correctly. 

The voltage of reading of the downstream oxygen sensor changes based on the air to fuel ratio. If it’s correct, the sensor should read about .45 volts. A reading over 1.2 volts indicates the exhaust stream is too rich. 

If this goes on for too long, the P0138 trouble code activates. The exact length of time varies from one model to the next. It can vary from as little as 10 seconds to as long as 4 minutes.

What Are The Symptoms Of The P0138 Code?

You may not notice any drivability symptoms with the P0138 OBD2 code. If you do, they’re likely to be fairly mild. The most common symptoms include:

  • Activation of the check engine light
  • Lean engine operation
  • Reduced fuel economy
  • Engine idles rough
  • Smell of gasoline from the exhaust

What Are The Causes Of P0138?

  • Faulty oxygen sensor
  • Shorted or corroded wiring around the oxygen sensor
  • Short in the oxygen sensor signal circuit
  • High fuel pressure (less likely)

How Serious Is The P0138 Code?

Driving with the P0138 code active for a long span of time could lead to damage of the internal components of the engine. While you can drive your car for a short while, you should make repairs as soon as possible.

How To Diagnose The P0138 Code

Tools You’ll Need:

The OBD2 scan tool can be used to diagnose the P0138 code
To diagnose the P0138 code, use the OBD2 scan tool

Method:

  1. Turn the key on without starting your engine. Use the OBD2 scanner to read the live data from all the terminals of both oxygen sensors. Compare the upstream data with the downstream readings. They should be very similar, with a slightly lower oxygen reading from oxygen sensor 2. 
  2. Visually inspect the downstream oxygen sensor for build-up or contaminants. Leaked oil or coolant can impair the sensor’s operation.
  3. Test oxygen sensor 2 using a digital multimeter. It should read around .3 volts as a reference signal. 
  4. Check the wiring around the downstream oxygen sensor for corrosion, damage, or loose connections. Replace and tighten as necessary.
  5. Visually inspect the exhaust system before the oxygen sensor for leaks. 
  6. Use a vacuum gauge to test the catalytic converter. Turn your car on and put it in park (or neutral, in a manual transmission) then put on the parking brake. Remove the vacuum hose and connect the vacuum gauge to the port. Start your car and let the engine idle until it reaches operating temperature. The gauge should read 18-22 Hg. Increase the RPMs. The vacuum reading should briefly drop then return to its earlier reading. If it doesn’t, the catalytic converter is likely clogged.
  7. Visually inspect the exhaust pipes for damage.

Common Mistakes To Avoid While Diagnosing The P0138 Code

Make sure you check the components around the oxygen sensor rather than assuming the sensor itself is the problem. A short in the wiring or clogged catalytic converter can also lead to a sustained over-rich mixture. Conduct a full diagnosis before you replace anything.

What Should You Do To Fix The Code P0138? 

  1. Repair any leaks damaged wires you find during your diagnosis.
  2. Clean the downstream oxygen sensor and the surrounding connections if you observed any build-up or spills during your diagnosis. Ensure all connections are secure once it’s reinstalled. 
  3. If the catalytic converter fails the test in step 5 of the diagnosis, cleaning it will likely remove the clog. You can buy catalytic converter from most auto parts stores. Use it following the directions on the bottle. 
  4. Clear the codes and test drive your car. If the P0138 trouble code comes back, replace the downstream oxygen sensor. 
  5. If this still doesn’t clear the code, there may be a more serious issue with either your catalytic converter or your internal electrical system. Take your car to a mechanic for further diagnosis. 
How to Fix the P0138 trouble code

Tips To Avoid P0138 In The Future

Clogs in the exhaust system can lead to a variety of different engine ailments. The easiest way to prevent clogs is to stay on top of changing your car’s fluids and other preventative maintenance. You can also avoid damage to the oxygen sensor by preventing leaks in the tubes and passages around it.

Read more: Autel TS408: An Excellent TPMS Service Tool Review

Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Diagnostic Tools and Auto Repair". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

Web: https://www.obdadvisor.comEmail: [email protected]
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