DTC code P0139 OBD2 basically indicates that the air-fuel ratio is not being adjusted as expected by the Oxygen Sensor (O2) signal or by the Engine Control module (ECM). Even the setting of this code may be due to the fact that after the engine is used normally, the ratio is not adjusted frequently. That is, the ECM determines that the O2 sensor has a slow response.
This code refers to Oxygen Sensor (O2) number 2 which is in bank number 1, i.e. the side of the engine where cylinder 1 is.