O2 Sensor Circuit High Voltage Bank 1 Sensor 2
The Powertrain Control Module (PCM) has detected that the voltage signal from the downstream oxygen sensor on Bank 1 (Sensor 2) is remaining excessively high for a prolonged period. This indicates either a continuous rich exhaust condition or a short to voltage within the sensor's wiring circuit.
While the vehicle will typically drive normally, ignoring this code can result in a failed emissions test and potential long-term damage to the catalytic converter if the engine is actually running rich.
Common symptoms
- Illuminated Check Engine Light
- Decreased fuel efficiency
- Failed emissions test
- Strong fuel odor from the exhaust
- Slightly rough engine idle
Likely causes
- Faulty Bank 1 Sensor 2 oxygen sensor
- Short to voltage in the O2 sensor signal wire
- Engine running excessively rich (e.g., leaking injectors, high fuel pressure)
- Corrosion or damage in the O2 sensor connector
- Faulty Powertrain Control Module (PCM)
What to check before replacing parts
Connect a scan tool and monitor the Bank 1 Sensor 2 live voltage data.
Look for: The voltage should fluctuate slightly around 0.45V to 0.8V; a steady reading above 0.9V to 1.2V confirms the high voltage fault.
Visually inspect the wiring harness and connector for the downstream O2 sensor.
Look for: Look for melted wires from exhaust heat, chafing, or water intrusion in the connector that could cause a short to voltage.
Disconnect the O2 sensor and measure the voltage on the signal wire from the PCM.
Look for: With the sensor unplugged, the scan tool or multimeter should read a baseline bias voltage (usually around 0.45V); if it reads 12V or 5V, there is a short in the wiring harness.
Review short-term and long-term fuel trim data on the scan tool.
Look for: Highly negative fuel trims indicate the engine is actually running rich, meaning the sensor is correctly reporting a rich condition rather than being faulty.
Test the O2 sensor ground circuit using a digital multimeter.
Look for: A poor ground can cause the sensor voltage to read artificially high; resistance between the sensor ground pin and the chassis should be less than 1 ohm.
This page explains the generic P0138 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
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