What does P0138 mean?
The bank 1, sensor 2 (downstream, after the catalytic converter) oxygen sensor measures leftover oxygen in the exhaust. Upstream sensors (sensor 1) help the computer adjust fuel; downstream sensors (sensor 2) mostly watch the catalytic converter.
The sensor voltage stayed high for too long. That can be a real rich condition or a sensor/wiring fault.
Codes are a starting point. They tell you which test failed, not which part to replace. Check for other stored codes and fix those in a sensible order.
Symptoms
- Check engine light
- Poor fuel economy
- Possible rough idle or hesitation with upstream sensor faults
- Failed emissions test
Common causes
- Exhaust really is rich (leaking injector, high fuel pressure)
- Signal wire shorted to voltage, sometimes to the heater feed inside the harness
- Sensor contaminated by coolant or oil
How to diagnose P0138
- Check fuel trims for a rich condition first.
- Unplug the sensor and see whether the signal stays high (points to a wiring short).
- Look at the sensor tip for white (coolant) or black oily deposits.
- Check for other codes, especially fuel trim (P0171-P0175) and misfire codes. Fix those first.
- Watch the sensor on live data. A healthy upstream narrowband sensor switches quickly between about 0.1 and 0.9 V at warm idle and 2,500 rpm.
How to fix it
- Repair exhaust leaks
- Fix the underlying lean/rich condition if fuel trims are off
- Replace the oxygen sensor if it is slow, stuck or dead
Estimated repair cost
Likely repairs for P0138 run from about $100 to $450, depending on the cause.
| Repair | Typical range (parts + labor) |
|---|---|
| Diagnostic fee (scan + testing) | $100 - $200 |
| Oxygen / A/F sensor replacement | $150 - $450 |
| Exhaust leak repair (gasket/flange) | $100 - $400 |
These are rough US estimates for an independent shop in 2026, based on typical parts prices and standard labor times. Dealers, big trucks, luxury brands and high-cost areas often run higher. They are not quotes. Use the repair cost estimator to adjust for your vehicle.
How serious is P0138?
Moderate severity
Usually drivable, but a bad upstream sensor can hurt fuel economy and, over time, the catalytic converter.
P0138 FAQ
What does P0138 mean?
P0138 stands for "O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)". The bank 1, sensor 2 (downstream, after the catalytic converter) oxygen sensor measures leftover oxygen in the exhaust.
Can I drive with a P0138 code?
Usually drivable, but a bad upstream sensor can hurt fuel economy and, over time, the catalytic converter.
How much does it cost to fix P0138?
It depends on the cause. The likely repairs for P0138 range from about $100 to $450 at an independent shop in the US, plus a diagnostic fee of roughly $100-$200. These are rough estimates; dealer prices and some vehicles run higher.
What is the most common cause of P0138?
The most common causes are: exhaust really is rich (leaking injector, high fuel pressure); signal wire shorted to voltage, sometimes to the heater feed inside the harness; sensor contaminated by coolant or oil.