Error P0138 – how to fix it?

Code definition P0138

O2 high voltage sensor circuit (Bank 1, sensor 2)

What code P0138 means

P0138 is a generic OBD-II code that indicates that the O2 sensor in Bank 1, Sensor 2 has no output voltage less than 1.2 volts for more than 10 seconds, indicating a lack of oxygen in the exhaust stream.

What causes code P0138?

• The engine control module (ECM) sees the O2 sensor voltage for sensor bank 1 of sensor 2 above 1.2 volts when the ECM has commanded the fuel for a target lean condition on that engine bank.
• The ECM detects a high voltage problem and turns on the Check Engine light.
• The ECM uses other O2 sensors to try to control fuel injection using their values.

What are the symptoms of the P0138 code?

• The engine may idle during a sensor test to correct the problem and may fail or stall.
• Engine control light comes on.
• Depending on the cause of the rich condition malfunction, there may be problems with engine operation.

How does a mechanic diagnose a P0138 code?

• Scan the codes and document the freeze frame data, then erase the codes to verify the fault.
• Monitor O2 sensor data to see if the voltage changes between low and high levels at a fast rate compared to other sensors.
• Check the O2 sensor connections and harness connections for corrosion at the connections.
• Check the O2 sensor for physical damage and fluid contamination.
• Check for exhaust leaks in front of the sensor.
• For additional diagnostics, perform point-of-care tests recommended by the manufacturer.

The most common errors when diagnosing code P0138.

• Follow these simple tips to avoid misdiagnosis:
• The O2 1 sensor for bank 1 can be used to diagnose the O2 2 sensor for bank 1, analyzing the performance of both sensors for comparison. The operation should be almost the same, except that sensor 2 should have a lower O2 reading because the catalytic converter must burn off excess fuel and oxygen.
• Check the O2 sensor for oil or coolant contamination from any oil leaks in the engine.
• Check that the catalytic converter is not damaged or clogged, which can cause erroneous sensor readings.

How serious is the code P0138?

• The O2 sensor output voltage can be caused by a faulty exhaust catalytic converter, which can cause a high O2 sensor output voltage.
• The ECM may not properly control the engine’s fuel/air ratio, leading to a clogged catalytic converter and excessive carbon buildup in an engine with faulty spark plugs.

What repairs can fix a P0138 code?

1. Replacement of the O2 sensor in sensor bank 1 of sensor 2.
2. Repair or replace the wiring or connection to the O2 sensor for sensor 2 bank 1.
3. Replace catalytic converter upstream of sensor
4. Injector leak repair

Additional notes on code P0138

The O2 sensor’s high-voltage state indicates a lack of oxygen in the exhaust gas or other related problems, such as a leaking fuel injector or a damaged catalytic converter inside.