Error P0138 – how to fix?

Code Definition P0138

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

What does the code P0138 mean

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

What causes the code P0138?

• The engine control module (ECM) sees the O2 sensor voltage for sensor bank 1 above 1.2 volts when the ECM has commanded fuel to a target lean state 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 with their values.

What are the symptoms of the P0138 code?

• The engine may be idling during the sensor test to correct the problem, and may fail or stall.
• The Check Engine indicator light will come on.
• Engine running problems may occur depending on the cause of the rich state fault.

How does the mechanic diagnose the P0138 code?

• Scan the codes and document the data from the frozen frame, then erase the codes to verify the fault.
• Monitor the O2 sensor data to see if the voltage is changing between low and high at a rapid rate compared to other sensors.
• Check the O2 sensor wiring and harness connections for corrosion at the connections.
• Check the O2 sensor for physical damage and fluid contamination.
• Check for exhaust gas leaks upstream of the sensor.
• For further diagnostics, spot tests recommended by the manufacturer should be performed.

The most common errors when diagnosing the P0138 code

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

How serious is the P0138 code?

• The O2 sensor output voltage may be caused by a faulty exhaust gas catalytic converter which may cause the high output voltage of the O2 sensors.
• The ECM may not properly control the fuel-to-air ratio of the engine, resulting in a clogged catalytic converter and excessive carbon build-up in an engine with dirty spark plugs.

What repairs can fix the code P0138?

1. Replacing the O2 sensor in bank 1 of sensor 2.
2. Repair or replace wiring or connection to O2 sensor for bank 1 of sensor 2.
3. Replace the catalytic converter upstream of the sensor
4. Repair the leaking injector

Additional notes regarding the P0138 code

The O2 sensor high voltage status 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.