P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
Updated November 12, 2026
The downstream oxygen sensor on bank 1 is reading higher voltage than expected. High voltage normally means rich exhaust, so it can be a real rich condition or a sensor that's shorted or contaminated.
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🔧 Want to check this yourself? An OBD2 scanner reads the code; models with live data also show the sensor readings used in the checks below. See code readers and scanners from $24.99 ↓ · Which scanner do I need?
Common causes
- A failed or fuel-contaminated downstream O2 sensor
- A signal wire shorted to voltage
- The engine running rich (leaking injector, high fuel pressure)
- Coolant or oil contamination of the sensor
Code meanings can vary slightly by manufacturer. Use these as a starting point and confirm with your vehicle's service information or a mechanic before replacing parts.
Symptoms
- Check engine light
- Usually no driveability change
- Sometimes poor fuel economy if the engine is truly running rich
Is it safe to drive?
Usually drivable short term, but find out whether the engine is running rich, since that wastes fuel and can damage the converter.
How to check it with a scanner
- Plug an OBD2 scanner into the port under the dashboard and read all stored codes and the freeze-frame data (the conditions when the code set).
- Check fuel trims in live data. Large negative trims suggest the engine really is rich.
- Watch sensor 2 voltage; a reading stuck above about 1 volt points to a short or a bad sensor.
- Inspect the harness for chafing against other wires and check the connector for moisture.
- After a repair, clear the code and drive normally for a few days to confirm it doesn't come back.
A basic code reader shows the code; scanners with live data let you watch sensor readings, which is what you need for most of the checks above. See OBD2 scanners under $100.
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Questions
What does P0138 mean?
P0138 is the generic OBD2 code for "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)." The downstream oxygen sensor on bank 1 is reading higher voltage than expected. High voltage normally means rich exhaust, so it can be a real rich condition or a sensor that's shorted or contaminated.
Is it safe to drive with P0138?
Usually drivable short term, but find out whether the engine is running rich, since that wastes fuel and can damage the converter.
How do I check P0138 myself?
Read the code and freeze-frame data with an OBD2 scanner, then: Check fuel trims in live data. Large negative trims suggest the engine really is rich. Watch sensor 2 voltage; a reading stuck above about 1 volt points to a short or a bad sensor. Inspect the harness for chafing against other wires and check the connector for moisture.
Related codes
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