P0138 Oxygen sensor circuit high voltage (bank 1, sensor 2)
What P0138 means
The downstream oxygen sensor on bank 1, after the catalytic converter, is reporting a voltage that stays too high. The engine computer cannot use it to check the converter and may see the exhaust as rich.
P0138 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Medium
- Typical repair cost
- 60–300 €
- Workshop time
- 0.3–1.5 h
- Repair difficulty
- Easy
- DIY possible
- Yes
- Check engine light
- Yes
- Affects emissions test
- Yes
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Check engine light on
- Usually no noticeable change in how the car drives
- Catalyst readiness monitor not completing
- Possible failed emissions test at inspection
Common causes
- Failing downstream oxygen sensor
- Signal wire shorted to voltage or to the heater circuit
- Sensor contaminated by oil or coolant
- Engine running rich
Can I keep driving with P0138?
You can keep driving, but have it checked within one to two weeks. The engine is rarely affected, but the catalyst check cannot run and the car may fail its emissions inspection.
What to check first
- Check fuel trims and the upstream sensor to rule out a rich-running engine.
- Inspect the harness for melted or fused wires.
- Unplug the sensor and check whether the reading changes; a reading that stays high points to a wiring short.
- Replace the downstream sensor if mixture and wiring are fine.
Related codes
- P0137 Oxygen sensor circuit low voltage (bank 1, sensor 2)
- P0139 Oxygen sensor circuit slow response (bank 1, sensor 2)
- P0136 Oxygen sensor circuit (bank 1, sensor 2)
- P0135 Oxygen sensor heater circuit (bank 1, sensor 1)
- P0134 Oxygen sensor circuit no activity detected (bank 1, sensor 1)
- P0133 Oxygen sensor slow response (bank 1 sensor 1)
- P0132 Oxygen sensor circuit high voltage (bank 1, sensor 1)
- P0131 Oxygen sensor circuit low voltage (bank 1, sensor 1)
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
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