Toyota P0136 or P0138 Code: Common Solutions

Troubleshooting a Toyota P0136 or P0138 Code

Trouble codes P0136 or P013 are fairly common problems on Toyota. Toyota has issued a service bulletin T-SB-0001-10 to address the P0138, P0158, or P0606 issue. The bulletin applies to the vehicles listed below. Specifically, these codes pertain to the oxygen sensors, which are critical in monitoring and optimizing the air-fuel mixture for efficient combustion. Understanding the Toyota P0136 or P0138 code, what each means, their possible causes and symptoms, and the potential fixes can help you address the issues effectively and maintain your Toyota’s performance and emissions standards.

NOTE: These vehicles use two different types of sensors: an air/fuel

An air/fuel ratio sensor is not the same as an oxygen sensor.

ratio sensor and an O2 sensor. Everyone refers to them both as oxygen sensors, but they are DIFFERENT. An air/fuel ration sensor is installed upstream from the catalytic converter and is used to control air/fuel mixtures. The O2 sensor is installed post-catalytic converter and is used just to confirm that the catalytic converter is working properly.

Toyota P0136 or P0138 Code: What they mean

• P0136 Code— The P0136 code stands for “Oxygen Sensor Circuit Malfunction (Bank 1, Sensor 2).” This code indicates that the ECM (Engine Control Module) has detected a problem with the oxygen sensor circuit in Bank 1, Sensor 2. Bank 1 refers to the side of the engine with the number one cylinder, and Sensor 2 is the downstream oxygen sensor, located after the catalytic converter.

• P0138 Code— The P0138 code signifies “Oxygen Sensor Circuit High Voltage (Bank 1, Sensor 2).” This code indicates that the ECM has detected an excessively high voltage signal from the oxygen sensor in Bank 1, Sensor 2. A high voltage signal typically means the sensor is detecting a rich air-fuel mixture, which has more fuel than air.

The Possible Causes of P0136 or P0138 Codes

• Faulty Air/Fuel Sensor or Faulty Oxygen Sensor— The most common cause of these codes is a malfunctioning air/fuel sensor. Over time, the sensor can become contaminated or fail, leading to incorrect readings.
• Wiring Issues— Damaged or frayed wires, poor connections, or corrosion in the oxygen sensor circuit can cause erroneous signals and trigger these codes.
• Exhaust Leaks— Leaks in the exhaust system, particularly near the oxygen sensor, can cause incorrect readings by allowing outside air to enter the exhaust stream.
• Faulty Catalytic Converter— A malfunctioning catalytic converter can affect the oxygen levels detected by the downstream sensor, leading to these codes.
• Engine Running Rich— If the engine is running too rich, it means there’s too much fuel and not enough air in the combustion mixture. This can be caused by issues such as a leaking fuel injector or a faulty fuel pressure regulator.

How to Diagnose a Toyota P0136 or P0138 code

In order for the post-cat sensor to read high voltage, there must be a rich mixture coming out of the cat, or the sensor itself is faulty. Start by checking the response from the post-cat sensor by purposely introducing a large vacuum leak. That should force the post-cat sensor to read lean. Then, force a rich condition. If the post-cat sensor responds properly, then the air-fuel ratio sensor isn’t properly controlling air/fuel mixtures, which would force a steady rich or lean condition in the post-cat sensor.

Test the air-fuel ratio sensor by checking the voltage readings. The air fuel ratio sensor should read between 3.35 and 3.70 volts and the post cat sensor should read 0.70 to 0.90 volts. If the air fuel ratio sensor readings are off, replace the sensor.If the post-cat sensor doesn’t respond to the vacuum leak and is in rich condition, replace it.

In most cases, shops discover that the problem is a faulty air-fuel ratio sensor, NOT a bad post-cat O2 sensor. Automatically replacing the post-cat O2 sensor simply because it is listed in the trouble code is a common DIY mistake. DON’T BE LIKE EVERYONE ELSE. Do the testing

Vehicles affected by Toyota service bulletin T-SB-0001-10

2005 – 2010 Avalon
2007 – 2009 Camry
2010 Camry
2008 Highlander
2009 – 2010 Highlander
2006-2010 RAV4
2007 – 2010 Sienna
2009 – 2010 Venza

©, 2019 Rick Muscoplat