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

What does P0138 mean?

The downstream oxygen sensor on bank 1 is stuck at high voltage, which looks like a constantly rich exhaust. Sensor 2 is the downstream sensor, mounted after the catalytic converter; it mainly watches how well the converter is working and has little effect on fuel mixture.

Generic OBD-II definition: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). Confirm applicability in your vehicle service information.

Common causes

• Signal wire shorted to a voltage source (sometimes the heater wire inside the sensor)
• Sensor failed or contaminated
• Engine running rich (leaking injector, high fuel pressure)

Symptoms

• Check engine light
• Possible fuel smell or poor economy if the engine really is rich

Can I keep driving with P0138?

Severity: Low Usually safe to keep driving, but get it checked.

If the check engine light is flashing or the car runs badly, stop driving and have it checked.

Typical fixes

• Unplug the sensor and see if the reading drops; if it stays high, suspect the wiring
• Check upstream sensor data and fuel trims for a real rich condition
• Replace the sensor if it is internally shorted