Engine Code: P0138 (O2 Sensor Circuit High Voltage Bank 1, Sensor 2)


Modern vehicles have oxygen sensors in them, also known as O2 sensors. Their overall purpose is to measure the amount of air in the exhaust system while the exhaust gases are coming out of the engine. The engine control unit is the computer which regularly monitors and communicates with these sensors.

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O2 sensors will tell the computer how much oxygen is in the exhaust. Based on this information, the sensors will be able to tell if there is too much or too little oxygen in the internal combustion chamber. The fuel and air mixture may be described as “lean” if there is too much oxygen and not enough fuel. But if there are less oxygen and too much fuel, the mixture is described as “rich.”

Engine Code P0138

The O2 sensors are placed in different areas in the exhaust system. The O2 Oxygen Sensor Circuit High Voltage Bank 1, Sensor 2 can usually be found within the catalytic converter. This particular sensor keeps track of the exhaust stream to see what the oxygen levels are.

However, if the voltage output of the sensor is not under 1.2 volts for over 10 seconds, this means the exhaust stream does not have any oxygen. As a result, the powertrain will generate trouble code P0138.

When this problem is occurring, the first symptom you will notice is the “Check Engine” warning light coming on. This could mean a lot of different things, so you’ll probably not do anything until you notice some physical symptoms.

These could be symptoms like rough idling, engine failing, the lean running of the engine, bad fuel economy, and engine lag. There could be stronger and thicker exhaust fumes coming out of the tailpipe too. All these symptoms should prompt you to want to investigate the problem further.

Read also: Trouble Code: P0741 (Torque Converter Clutch Circuit Performance or Stuck Off)

If you or your mechanic uses a diagnostic scanning tool on the powertrain and it generates engine code P0138, then you know the problem is with O2 Oxygen Sensor Circuit High Voltage Bank 1, Sensor 2.

This sensor may be defective from corrosion, but there could also be defective wiring, high fuel pressure, faulty oxygen sensor connection, or a short. You will not be able to know the exact cause until you check these areas of the exhaust system.


In most scenarios, you’ll need to replace the Oxygen Sensor. A new sensor is priced at around $250 to $300. The labor will be the biggest expense because that will be about $140 to $180. The part itself will cost about $100 to $120.



    • First, the problem that’s causing the check engine light will actually need to be fixed. Then you can use an OBD2 scan tool to reset the light. Sometimes, disconnecting the battery for a few minutes or simply letting the light reset by itself (as the car’s computer rechecks the problem area) will also work.


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