P0138
O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
The downstream oxygen sensor signal is fixed at a high voltage, indicating either a continuously rich downstream reading (less common, since post-cat exhaust should be more stable) or a short to supply voltage in the signal circuit.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: easy
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Common causes
Ordered by how often each turns out to be the cause.
- Signal wire shorted to supply voltageCommon
- Downstream sensor failedPossible
- Connector pins corroded - high contact resistanceLess likely
Parts involved
- Oxygen Sensor Downstream
- Oxygen Sensor Wiring
P0138 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.