P0138 means O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). The downstream oxygen sensor on bank 1 is reading higher voltage than expected, which reads as a constantly rich exhaust or a short. It is a generic OBD-II code, so the definition is the same on any make, but the most likely cause varies by engine; confirm with a scan tool and the service information for your vehicle before buying parts.
What P0138 means
Oxygen sensors measure the oxygen left in the exhaust. Sensor 1 sits before the catalytic converter and is used for fuel control; sensor 2 sits after it and mainly monitors the converter. Most sensors have a heater so they start working soon after a cold start. A traditional zirconia sensor switches between about 0.1 volt (lean) and 0.9 volt (rich); many newer upstream sensors are wide-band air-fuel sensors that report differently, so check which type your engine uses.
P0138 symptoms
- Check engine light
- Usually no drivability change
- Fuel smell if the engine really is rich
Common causes of P0138
- Failed sensor
- Short to voltage in the signal wire
- Real rich condition from a leaking injector or high fuel pressure
- Sensor contaminated by oil or coolant
How to diagnose P0138
- Check fuel trims for a rich condition
- Inspect the wiring for shorts
- Watch the sensor signal on a scan tool
Can you drive with P0138?
Yes, but check for a rich condition that could damage the catalytic converter.
P0138 tips
A high reading can also come from a sensor contaminated by oil or coolant, so look for signs of oil burning or a coolant leak before fitting a new sensor.
Will P0138 clear itself?
If the fault stops, the computer turns off the check engine light by itself after several drive cycles without seeing it again, and the code stays in memory as history for a while. Clearing P0138 with a scan tool after a repair is fine, but if the cause is still there the code returns, sometimes within one drive. Disconnecting the battery also clears codes on most vehicles, but it erases learned fuel trims and idle settings as well.
Related codes
| Code | Meaning |
|---|---|
| P0137 | O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) |
| P0141 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2) |
| P0172 | System Too Rich (Bank 1) |
Guides that help
When it is more than a sensor
Oxygen sensor codes are fixed with a sensor, wiring, a fuse or an exhaust leak repair. An engine burning oil or coolant contaminates sensors quickly; if new sensors keep failing, check for blue or white exhaust smoke and a head gasket leak, and compare engines by code on our engines page if the engine is worn out.
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