P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)

What does P0138 mean?

The downstream sensor normally shows a fairly steady reading once the catalytic converter is working. P0138 means its voltage stayed high, reporting a rich exhaust, even when the ECM commanded a leaner mixture.

The cause may be the sensor or its circuit, or a real rich running condition upstream. The exact threshold varies by make and model.

Common symptoms

• Check engine light on
• Often no obvious symptoms
• Hesitation or rough running if the engine really runs rich
• Lower fuel economy

Common causes

The causes below are the ones commonly seen with P0138. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• A faulty downstream oxygen sensor
• Wiring or connector problems, such as a short in the sensor circuit
• A real rich condition from the fuel system

What to check with a scanner

• In Live data, watch the bank 1 sensor 2 voltage with the engine fully warm. A healthy narrow band sensor moves between low and high readings rather than sitting still.
• Read freeze frame data to see the engine temperature, load and rpm when the code was stored.
• Compare short term and long term fuel trim. Large trims point to a real lean or rich condition rather than a sensor fault.
• Note any other codes, especially heater circuit, misfire or catalyst codes, before clearing anything.

How to read and clear P0138 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0138 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0138 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0138.

When to see a mechanic

Get professional help if any of these apply:

• The code returns right after clearing
• The engine runs rough, misfires or the check engine light flashes
• Wiring near the exhaust looks melted or chafed
• You need the sensor tested with a meter or scope

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Related trouble codes

See all OBD2 trouble code guides

Related guides

Check engine light on: what to do · How to read an OBD2 code (P, B, C, U) · Stored, pending and permanent codes and freeze frame · OBD2 readiness monitors explained · OBD2 glossary · OBD2 by car brand