P0138 OBD-II Trouble Codes: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

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Cost of diagnosing the P0138 code

P0138 is a diagnostic trouble code (DTC) for "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic for $114.99 . Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.0 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0138 code definition

O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)

What the P0138 code means

P0138 is the OBD-II generic code indicating the O2 sensor for bank 1 sensor 2 fails to have a lower voltage output below 1.2 volts for more than 10 seconds indicating a lack of oxygen in the exhaust stream.

What causes the P0138 code?

  • The engine control module (ECM) sees the voltage of the O2 sensor for bank 1 sensor 2 above 1.2 volts when the ECM has commanded the fuel to a targeted lean condition on that bank of the engine.

  • The ECM detects the voltage high problem and turns on the Check Engine Light.

  • The ECM uses other O2 sensors to try and control the fuel injection with their values.

What are the symptoms of the P0138 code?

  • The engine may run lean during the testing of the sensor to correct the problem and may hesitate or misfire.

  • The Check Engine Light will be illuminated.

  • You may have engine running problems depending on failure cause of the rich condition.

How does a mechanic diagnose the P0138 code?

  • Scans codes and documents freeze frame data and then clears codes to verify failure.

  • Monitors O2 sensor data to see if the voltage is switching back and forth between low and high at a fast rate compared to other sensors.

  • Checks the O2 sensor wiring and the harness connections for any corrosion in the connections.

  • Checks the O2 sensor for any physical damage or fluid contamination.

  • Checks for exhaust leaks before the sensor.

  • Follows the manufacturer's specific pinpoint tests for further diagnosis.

Common mistakes when diagnosing the P0138 code

Follow these simple guidelines to prevent misdiagnosis:

  • O2 sensor 1 for bank 1 can be used to diagnose O2 sensor 2 for bank 1 by looking at both sensors operation for comparison. The operation should be nearly the same, except sensor 2 should have a lower O2 reading since the catalyst should burn off the excess fuel and oxygen.

  • Check the O2 sensor for oil or coolant contaminants from any engine leaks.

  • Check the catalyst for damage or it being clogged that can cause erratic sensor readings.

How serious is the P0138 code?

  • The voltage output from the O2 sensor may be due to the exhaust catalyst being broken apart, which can cause the O2 sensors to give high output voltages.

  • The ECM may not control the fuel-to-air ratio of the engine properly, leading to a clogged catalyst and excessive carbon buildup in the engine with fouled spark plugs.

What repairs can fix the P0138 code?

Additional comments for consideration regarding the P0138 code

The high voltage condition from the O2 sensor is indicating a lack of oxygen in the exhaust or other related problems, such as a leaking fuel injector or a broken up catalyst inside.

Need help with a P0138 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

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Related questions

Getting a check engine light, P0138 error: "O2 Sensor circuit High voltage (Bank 1, sensor 2)"
Hi, thanks for writing in. I wouldn't worry about it. The only purpose of the downstream O2 sensor is to monitor the health of the catalytic converter. You have four O2 sensors. The two upstream sensors monitor fuel mixture (Bank...
Why is it still misfiring?
Hi there. Check the Exhaust Gas Recirculation (https://www.yourmechanic.com/services/exhaust-gas-recirculation-egr-valve-replacement) valve on the engine. It seems to be sticking on when the engine is running at low idle and causing a misfire as the old exhaust that is burnt is reentering the...
Help! P0138 came back after I put engine cleaner in my tank and have replaced the entire exhaust system from headers to muffler.
The code P0138 (https://www.yourmechanic.com/article/p0138-obd-ii-trouble-codes-o2-sensor-circuit-high-voltage-bank-1-sensor-2-by-jay-safford) is on for two possible different situations. Either the oxygen sensor has shorted out on the exhaust system or the wiring from the battery to the sensor has a short or break in it. Check the...

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