Code Definition

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

System: Powertrain  |  Type: Generic (OBD-II Standard)  |  Make: Generic

About P0138

P0138 is a generic (obd-ii standard) powertrain diagnostic trouble code. It belongs to the P-prefix family of codes, which covers engine, transmission, emissions, or fuel-system faults.

When P0138 is stored, the module has detected a signal or supply voltage higher than the expected operating range; a bank-specific fault, meaning it applies only to one cylinder bank of a multi-bank engine; and a sensor-related fault — the reported value from the named sensor is out of spec.

Scanning & clearing: A generic OBD-II scan tool will usually read and clear powertrain (P) codes. For live data, freeze-frame capture, and bi-directional tests you may need a more capable scan tool that supports the vehicle's ECM protocol.

Because P0138 is a generic OBD-II code, its base definition is shared across manufacturers. The specific diagnostic steps, reference values, and repair procedures can vary by vehicle, and the TaT Diagnostic Assistant includes verified procedures submitted by working technicians.

Code Information

The P0138 fault code is activated when the Engine Control Module (ECM) identifies an abnormally high voltage output from the oxygen sensor in Bank 1, Sensor 2. This sensor is generally located downstream in the exhaust system, after the catalytic converter. A high voltage signal from this sensor is typically indicative of a rich air-fuel mixture or a reduction in the oxygen content in the exhaust stream. This could be due to various factors including a malfunctioning sensor, issues with the wiring or connectors, or problems with the catalytic converter's efficiency. The ECM uses this downstrea...

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