P0139 means O2 Sensor Circuit Slow Response (Bank 1 Sensor 2). The downstream oxygen sensor on bank 1 is responding too slowly when the fuel mixture changes. 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 P0139 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.
P0139 symptoms
- Check engine light
- Usually no drivability change
Common causes of P0139
- Aging or contaminated sensor
- Exhaust leak near the sensor
- Heater circuit fault
How to diagnose P0139
- Check sensor response on a scan tool during throttle snaps
- Check the heater circuit
- Check for exhaust leaks
Can you drive with P0139?
Usually yes for a while, but fix it to protect fuel economy and the catalytic converter.
P0139 tips
Slow downstream sensor response often comes with age; replacing an original high-mileage sensor usually fixes it.
Will P0139 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 P0139 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) |
| P0138 | O2 Sensor Circuit High Voltage (Bank 1 Sensor 2) |
| P0141 | O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2) |
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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