P0138 High Oxygen Sensor Circuit O2 (B1S2)

OBD-2 Technical Description - P0138

O2 Oxygen Sensor Circuit High Voltage (Bank1, Sensor2)

P0138 is a generic OBD-II code indicating that the O2 sensor for bank 2 sensor 1 does not have a lower voltage output below 1,2V for more than 10 seconds, indicating a lack of oxygen in the exhaust stream.

What does the fault code mean P0138?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to OBD-II equipped vehicles. Although general, specific repair steps may differ depending on the brand / model.

The heated oxygen sensor (2) located at the rear of the catalytic converter provides an output signal related to the oxygen storage capacity of the catalytic converter. Bank 1 is the side of the engine that contains cylinder #1.

The Ho2S 2 signal is less active than the front oxygen sensor signal. This code is set when the HO2 sensor voltage exceeds 999 mV for more than 2 minutes (time depends on the model. May be up to 4 minutes)

Symptoms

There may be no noticeable symptoms except for the MIL illumination. Possible high fuel pressure can overload the system.

• The engine may run lean during the sensor test to correct the problem and may oscillate or misfire.
• The Check Engine light will come on.
• You may experience engine performance issues depending on the cause of the rich condition failure.

Causes of error P0138

The P0138 code may mean that one or more of the following events have occurred:

• Defective O2 sensor
• Short circuit to battery voltage in the O2 sensor signal circuit
• High fuel pressure (unlikely)
• The engine control module (ECM) sees that the O2 sensor voltage for bank 2 sensor 1 is greater than 1,2 V when the ECM commands a target lean fuel on that bank of the engine.
• The ECM detects a high voltage problem and illuminates the Check Engine light.
• The ECM uses other O2 sensors to try and control fuel injection with their values.

Possible solutions

Here are some possible solutions:

• Replace O2 sensor
• Repair the short to voltage in the o2 sensor signal circuit.

How does a mechanic diagnose a P0138 code?

• Scans data freeze frame codes and documents and then clears codes to confirm failure.
• Monitors O2 sensor data to see if voltage switches between low and high at a high rate compared to other sensors.
• Checks O2 sensor wiring and harness connections for corrosion at the connections.
• Checks the O2 sensor for physical damage or fluid contamination.
• Checks for exhaust leaks ahead of the sensor.
• Follows manufacturer's specific spot tests for further diagnosis.

Common Mistakes When Diagnosing Code P0138

Follow these simple guidelines to prevent misdiagnosis:

• The Bank 2 O1 Sensor 1 can be used to diagnose the Bank 2 O2 Sensor 1 by comparing the performance of both sensors. Operation should be much the same, except sensor 2 should have a lower O2 reading as the catalyst has to burn off excess fuel and oxygen.
• Check the O2 sensor for oil or coolant contamination from any engine leaks.
• Check the catalytic converter for damage or blockage, which may cause erroneous sensor readings.

How serious is the P0138 code?

• The output voltage of the O2 sensor may be due to the destruction of the catalytic converter, which can cause the O2 sensors to produce a high output voltage.
• The ECM may not properly control the engine's fuel/air ratio, resulting in catalytic converter fouling and excessive carbon deposits in an engine with dirty spark plugs.

What repairs can fix code P0138?

• Bank 2 Sensor 1 O2 Sensor Replacement
• Repair or replace wiring or connection to O2 sensor bank 1 sensor 2
• Replacing the catalyst in front of the sensor
• Leaking nozzle repair

Additional comments to consider regarding code P0138

A high voltage condition from the O2 sensor indicates a lack of oxygen in the exhaust or other related problems such as a leaking fuel injector or a broken catalytic converter inside.

How to Fix P0138 Engine Code in 3 Minutes [2 DIY Methods / Only $8.99]

Need more help with your p0138 code?

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