Oxygen Sensor Slow Response (Bank 1 Sensor 2)
The engine computer is not seeing the rear oxygen sensor change fast enough. This usually means the sensor is slow, the exhaust has a leak, or there is a wiring problem. It can also happen if the catalytic converter or fuel system is affecting exhaust readings.
You can usually drive for a short time, but the problem should be diagnosed soon. If the check engine light is flashing, the engine runs badly, or fuel mileage drops sharply, stop driving and get it checked right away.
P0139 means the powertrain control module has detected that Bank 1 Sensor 2, the downstream oxygen sensor after the catalytic converter, is switching voltage too slowly. The sensor signal is compared to expected changes during closed-loop operation, and the PCM expects a faster response than it is seeing. This can be caused by a worn oxygen sensor, exhaust leaks, damaged wiring, or less commonly a catalytic converter or engine running issue.
No. The sensor is common, but exhaust leaks, wiring faults, and engine problems can also cause this code.
It is the downstream oxygen sensor on the side of the engine with cylinder 1, located after the catalytic converter.
Sometimes, but not always. The sensor should be tested and the exhaust system checked first so you do not replace parts unnecessarily.
Yes, but it is less common than a sensor, wiring, or exhaust leak problem. Converter issues should be confirmed with testing.