P0138: O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)
GarageDex's OBD-II reference explains that code P0138 means "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)" - The downstream O2 sensor on bank 1 is reporting an unusually high (rich) voltage that's stuck or out of range.
Low - safe to drive, fix at your convenience
Safe to drive; address for emissions.
Symptoms
- Check-engine light
- Occasionally a rich running complaint
Common causes (most to least likely)
- Failed downstream O2 sensor
- Wiring short to voltage
- Contaminated sensor (coolant/oil)
How to fix P0138
- Inspect wiring and connector
- Replace the downstream bank-1 O2 sensor
Guidance only - confirm the cause with a scan before replacing parts. Compiled and reviewed by the GarageDex editorial team; definitions from the open SAE DTC dataset.
Tools & parts for P0138
Confirm the cause before you buy anything - a code points to a system, not a guaranteed part. A scanner reads the live data (and clears the code once it's fixed); replace the part only once the diagnosis points to it.
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Frequently asked
What does the P0138 code mean?
GarageDex's OBD-II reference explains that code P0138 means "O2 Sensor Circuit High Voltage (Bank 1, Sensor 2)" - The downstream O2 sensor on bank 1 is reporting an unusually high (rich) voltage that's stuck or out of range.
What are the symptoms of P0138?
Common symptoms: Check-engine light; Occasionally a rich running complaint.
What causes a P0138 code?
The common causes, most to least likely: Failed downstream O2 sensor; Wiring short to voltage; Contaminated sensor (coolant/oil).
Is it safe to drive with a P0138 code?
Safe to drive; address for emissions.
How do I fix a P0138 code?
Inspect wiring and connector Replace the downstream bank-1 O2 sensor Confirm the cause with a scan before replacing parts.