Emission controls · Powertrain · generic (SAE-defined)
P0138: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)
P0138 means the post-catalytic-converter oxygen sensor on bank 1 is reading higher voltage than it should, indicating a stuck-rich signal or a short to voltage.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0138 sets
The powertrain control module (PCM) monitors the bank 1 sensor 2 oxygen (O2) sensor after the catalytic converter. It may command a lean event to see whether the sensor voltage drops. If the signal stays above its expected range for a calibrated time, the PCM records P0138. Values are manufacturer-specific.
Common causes of P0138
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Failed bank 1 sensor 2 oxygen sensor
- Signal wire shorted to the heater supply or another voltage source
- Engine running rich from fuel system problems
- Sensor contaminated by oil or fuel
- Water intrusion in the connector
P0138 symptoms
- Check engine light on
- Usually no drivability symptoms
- Possible poor fuel economy or fuel odor if the engine is rich
How P0138 is diagnosed
- Scan for codes; rich fuel trim or misfire codes should be diagnosed first.
- Inspect the downstream sensor wiring for melted or fused insulation.
- On live data, create a lean condition (for example, a brief vacuum leak or deceleration); the reading should drop.
- With the sensor unplugged and key on, check for voltage on the signal wire that indicates a short.
- Check fuel trims and fuel pressure if the engine appears truly rich.
Typical fixes for P0138
- Replace the bank 1 sensor 2 oxygen sensor
- Repair a short to voltage in the sensor harness
- Correct a rich-running condition
- Clean or replace a corroded connector
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Related codes & warning lights
P0138 FAQ
What does code P0138 mean?
P0138 means the post-catalytic-converter oxygen sensor on bank 1 is reading higher voltage than it should, indicating a stuck-rich signal or a short to voltage. The standard definition is "O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)."
Can I drive with a P0138 code?
Usually yes. Drivability is rarely affected, though if the engine is actually running rich, fuel economy suffers and the converter can be stressed, so do not put it off indefinitely.
Is P0138 a common oxygen sensor failure?
It is a fairly common downstream sensor code, especially on older sensors, where an internal fault causes the reading to stay high even when the exhaust is not rich.
Should I check the engine for running rich?
Yes. A rich mixture would also raise the downstream reading, so check fuel trims and look for misfires or leaking injectors before replacing the sensor.
Keep researching
More emission controls codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.