P0138 — O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Low severityWhat it means
P0138 is stored when the engine control unit sees a fault involving the downstream oxygen sensor on bank 1 (sensor 2). The bank 1 downstream oxygen sensor sits after the catalytic converter and is used mainly to monitor how well the converter is working. A “high” reading means the signal is pinned near the top of its range — often an open circuit, a lost ground, or a short to voltage.
Common symptoms
- The live reading sits abnormally high or maxed out
- Often only a check engine light
- A possible failed emissions test
- Rarely any driveability change
Likely causes
- Open signal or ground wire
- Signal wire shorted to a voltage supply
- Aged downstream oxygen sensor
- An exhaust leak after the converter
What to check first
- Check for an open circuit or missing ground at the sensor connector
- Compare the upstream and downstream signals in live data
- Inspect the rear exhaust and the sensor wiring
Can you keep driving?
Generally safe to drive; repair to restore catalyst monitoring and pass emissions.
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