Fault code P0138 – O2 sensor circuit high voltage (Bank 1, Sensor 2)

Fault code P0138 is called “O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)” but in different programs it may be called differently. This fault designation applies to all vehicles equipped with OBD-II.

Technical description and explained code P0138

This Diagnostic Trouble Code (DTC) is a generic powertrain code. Error P0138 is considered a general code because it applies to all makes and models of vehicles. Although the specific repair steps may vary slightly depending on the model.

Fault code P0138 – O2 sensor circuit high voltage (Bank 1, Sensor 2)

The catalytic converter is used to reduce harmful emissions. To ensure proper operation, an O₂ sensor is located behind the catalytic converter. This monitors the oxygen content of the exhaust gases after the catalytic converter.

The PCM (transmission control module) then compares the readings after the test with the readings before the test. To determine if the catalytic converter is working properly. Code P0138 refers to a malfunction of the oxygen sensor O₂. Which is after the catalytic converter on block 1 sensor 2. The error indicates the signal voltage is too high.

The sensor O₂ is a four wire sensor. Two wires are for the heating element and two wires are for the sensor. The heater element must have battery voltage on one lead with the engine off. And ground on the other.

The PCM supplies 0.5 volt voltage to the O₂ sensor, which the sensor varies depending on the oxygen content of the exhaust. The PCM also provides the grounding of the sensor.

Changes in the oxygen content, cause the resistance of the O₂ sensor to change. This change in resistance affects the 0.5 volt voltage supplied by the PCM. It can range from 0.1 to 0.9 volts. A value of 0.1 indicates complete depletion and 0.9 indicates complete enrichment.

The code P0138 means that the voltage in the signal circuit was higher than normal. It is worth noting that this sensor switches slower and less frequently than the pre-catalyst sensors.

Symptoms of vehicle malfunctions

The main signal that an error P0138 has occurred is the Malfunction Indicator Lamp (MIL) is also known as the CheckEngine Light.

It can also be warning signs such as:

  1. The Check Engine light comes on (malfunction indicator light).
  2. You may notice lack of power and ignition skips in the engine.
  3. Increased fuel consumption.
  4. The engine is running on a rich mixture.

Fault P0138 may indicate a failure of the oxygen sensor, so the ECM cannot adjust the air-fuel mixture ratio properly. This may result in increased fuel consumption, as well as problems with the vehicle’s engine and exhaust system performance.

Factors that can cause this error code

The error code P0138 can mean that one or more of the following problems have occurred:

  • Defective oxygen sensor O₂ (Bank 1, Sensor 2).
  • Oxygen sensor cable O₂ may be in contact with the exhalation element.
  • The engine is running on rich mixture (if other codes are present).
  • Holes and exhaust leaks cause the engine PCM to over-enrich, resulting in an abnormally high reading Bank 1, Sensor 2.
  • Possible short in signal circuit.
  • PCM not working properly.

How to fix or reset OBD-2 code P0138

Some suggested steps for troubleshooting and fix the error code P0138:

  1. Read all data and error codes stored in the PCM memory using an OBD-II scan tool.
  2. Observe the oxygen sensor voltage in real time using the scan tool. To find out if the sensor is working properly.
  3. If necessary, check the engine coolant temperature sensor operation.
  4. Check the electrical wires going to the oxygen sensor for shorts and damage.
  5. Measure fuel pressure using a pressure gauge.
  6. Determine if the control module (PCM) needs reprogramming or replacement.

Diagnose and repair of problems

When starting the diagnosis of fault P0138, it is necessary to observe the condition of the exhaust system components. Check if the wiring harness making contact with the exhaust, etc. is damaged. Start the engine and check the voltage of O₂ Bank 1 of Sensor 2.

On the sensors after the catalyst you may need to increase the idle speed until the sensor O₂ starts to switch if the sensor is slow. If increasing the idle speed causes the voltage on the sensor to drop, replace the sensor due to slow operation.

If the oxygen sensor hangs high or near 1 volt after starting the engine and warming up to operating temperature. Voltage does not drop or change, turn off engine and replace sensor.

Disconnect the Sensor 2 Bank 1 O₂ and check that the grounding is secure on both the heating element and the sensor. Also make sure that good battery voltage is supplied to the heating element.

No heater operation can cause sensor failure, but there are usually fault codes associated with it. If the ground is good and battery voltage is present, connect the sensor signal lead to the ground wire.

The diagnostic tool should now show low or near zero voltage. If it does, the wiring is fine. Then you need to replace the sensor. Make sure there is no contact with hot parts of the exhaust system. Make sure the wiring harness is well connected.

After checking the sensor wiring, check the wiring harness at the PCM connector. It may be necessary to secure the sensor signal wire O₂ near the PCM in an easily accessible location. So that it can be repaired after this test to clear the P0138 fault.

If the O₂ sensor reading drops, then the wiring problem is not visible. Open the harness and visually inspect the wiring for problems with the signal wire. Or run another signal wire by cutting and leaving the problem wire in the harness. But if the result is the same, the PCM may be to blame.

On which vehicles does this problem occur most frequently

Fault code P0138 can occur on different vehicles but there are statistics on which brands this occurs most often. Here is a list of some of them:

  • Chevrolet (Aveo, Captiva, Cruze, Epica, Lacetti, Rezzo)
  • Chrysler (300C, Pacifica, Sebring, Voyager)
  • Citroen (C5)
  • Daewoo (Matiz, Nexia)
  • Dodge (Caravan, Dakota, Neon, RAM, Stratus)
  • Ford (Escape, Explorer, Focus, Mondeo)
  • Honda (Accord, CR-V, Civic, Odyssey)
  • Hover
  • Hyundai (Accent, Getz, Santa Fe, Tucson)
  • Infiniti
  • Jeep (Grand Cherokee, Wrangler)
  • Kia (Rio, Sorento, Sportage)
  • Lexus (RX350)
  • Lifan (Breez, Solano)
  • Mazda (3, 6, CX-7)
  • Mercedes-Benz
  • Mitsubishi (Lancer, Outlander)
  • Nissan (Almera, Murano, Pathfinder, Patrol, Primera, Qashqai, Teana, X-Trail)
  • Opel (Antara, Astra, Corsa)
  • Peugeot (206, 308, 407)
  • Pontiac (Vibe)
  • Renault (Duster, Fluence, Logan, Sandero)
  • Saab
  • Skoda (Octavia)
  • Ssangyong
  • Subaru
  • Suzuki (Grand Vitara)
  • Toyota (Avensis, Camry, Highlander, Prado, Prius, RAV4, Venza)
  • Volkswagen
  • GAZelle (Next)
  • TagAZ
  • VAZ

Fault code P0138 can sometimes be found with other errors. The most common are the following: P0038, P0108, P0113, P0118, P0123, P0132, P0134, P0136, P0137, P0139, P0140, P0141, P0158, P0170, P0172, P0325, P0340, P0351, P0447, P0501, P0606, P1167, P2096, P2253.


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