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

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?

• Replacing the O2 sensor for bank 1 sensor 2
• Repairing or replacing the wiring or connection to the O2 sensor for bank 1 sensor 2
• Replacing the catalyst in front of the sensor
• Repairing a leaking injector

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?

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