Ford P0138 OBD-II Trouble Code Definition: P0138 HO2S12 Circuit High Voltage P0138 Ford OBD-II Trouble CodeDescription: OBD Trouble Code P0138 O2 Sensor Circuit High Voltage What does the code mean? OBD-ii Code P0138 definition: The Heated Oxygen Sensor (2) located rear of catalytic converter produces an output signal relative to oxygen storage capacity of catalytic converter. […]