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.
General code reference. Confirm the definition and diagnostic procedure for your vehicle's year, model and engine. A stored code alone does not identify a failed part.
Affected components
- Oxygen sensor downstream
- Oxygen sensor wiring
Possible causes to check
These are possible checks, not a ranked diagnosis. Follow the vehicle manufacturer's test procedure before replacing parts.
- Signal wire shorted to supply voltage
- Downstream sensor failed
- Connector pins corroded - high contact resistance
Possible symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Diagnostic notes
- The malfunction indicator lamp may illuminate
- This code can affect emissions
- A drive cycle may be required after repair
Related codes
Sources
Check each reference's vehicle and model-year scope. General background links do not establish which fault is most likely on your vehicle.
Diagnostic information is provided for reference only. Confirm the code with a compatible scan tool and follow the vehicle manufacturer's service procedures before replacing components.