Code Definition

P0138 — Oxygen 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

Bank 1 sensor 2 is the downstream oxygen sensor mounted after the catalytic converter, where it monitors catalyst efficiency and helps trim fuel. DTC P0138 sets when the engine control module sees the voltage from this sensor stay abnormally high, typically above about 1.0V or pegged near the top of its range. A high voltage reading usually means a genuinely rich exhaust, a signal circuit shorted to a voltage source, or coolant and contamination inside the sensor. When the code is set the ECM may skew the fuel trims, and the driver can notice poor fuel economy, a sulphur smell, or a check engi...

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