P0138Powertrain
Powertrain · SAE
O2 Sensor Bank 1 Sensor 2 – High Voltage
Meaning
The downstream oxygen sensor on Bank 1 reports a voltage consistently above the normal range (above ~0.9 V). This indicates rich exhaust gas past the catalyst or a faulty sensor.
Common causes
- Defective downstream O2 sensor (signal stuck high)
- Persistently rich fuel mixture exceeding the catalyst's oxidation capacity
- Degraded or depleted catalytic converter
- Signal wire short to reference voltage or to +12V
- Fuel contamination near the sensor
Symptoms
- MIL (Check Engine) warning light on
- Possible increase in fuel consumption
- Elevated CO and HC emissions on emissions test
- No noticeable driveability symptom in most cases
- Possible pungent exhaust smell
Severity
Moderata: The vehicle is generally driveable, but the emissions control system is compromised and the catalytic converter may be damaged if the condition persists.
What to do
- Read DTCs and live data with a scan tool, checking downstream sensor voltage with the engine at operating temperature.
- Visually inspect the wiring harness and connector of sensor B1S2 for shorts, corrosion, or damage.
- Verify sensor supply voltage and ground reference against manufacturer specifications.
- Check long-term fuel trim (LTFT) to determine whether the engine is operating in a sustained rich condition.
- Replace the downstream O2 sensor B1S2 if electrical tests confirm sensor failure; evaluate catalytic converter replacement if a persistent rich mixture is the root cause.