What it means
The module detects that the downstream oxygen sensor on bank 1 responds too slowly under the monitored conditions.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Bank 1 Sensor 2 is the post-catalyst oxygen sensor, and the control module uses its exhaust reading when checking catalyst operation. P0139 sets when the module observes that this sensor takes too long to respond under monitored conditions, which can point to a sluggish sensor, circuit problem, or an exhaust or mixture issue.
Common causes
Most likely first.
- Engine running rich (leaking injector, high fuel pressure)
- Sensor signal wire shorted to voltage
- Downstream O2 sensor failed
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
How to diagnose it
- Review freeze-frame data and monitor conditions to determine when the slow response was detected.
- Inspect Bank 1 Sensor 2, its connector, and wiring for heat damage, corrosion, loose terminals, or a signal wire shorted to voltage.
- Check for an exhaust leak near the downstream sensor and assess whether the engine is running rich using available fuel-trim data.
- Graph Bank 1 Sensor 2 live data during a warm-engine drive or supported monitor test and compare its response with the upstream sensor.
- Test the sensor signal and heater circuits, then check for causes of a rich condition such as injector leakage or excessive fuel pressure using the manufacturer's specification.
Common fixes
- Repair damaged or shorted wiring and poor connector contacts at Bank 1 Sensor 2.
- Repair an exhaust leak near the downstream sensor.
- Correct a confirmed rich-running cause, such as a leaking injector.
- Replace Bank 1 Sensor 2 if it responds slowly despite sound wiring and no exhaust or mixture fault.
Can I keep driving?
You can usually drive cautiously if the engine runs smoothly, but fuel use may rise and the vehicle may fail an emissions test. If rough running or a flashing check-engine light develops, stop driving because continued misfiring or rich running can damage the catalyst.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.