P0139 Lexus Code - Oxygen Sensor Circuit Slow Response Bank 1 Sensor 2

In order to obtain a high purification rate of the Carbon Monoxide (CO), Hydrocarbon (HC), and Nitrogen Oxide (NOx) components in the exhaust gas, a Three-Way Catalytic Converter (TWC) is used. For the most efficient use of the TWC, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel level. For the purpose of helping the Engine Control Module (ECM) to deliver accurate air-fuel ratio control, a heated oxygen sensor is used.
The heated oxygen sensor is located behind the TWC and detects the oxygen concentration in the exhaust gas. Since the sensor is integrated with the heater that heats the sensing portion, it is possible to detect the oxygen concentration even when the intake air volume is low (the exhaust gas temperature is low).
When the air-fuel ratio becomes lean, the oxygen concentration in the exhaust gas is high. The heated oxygen sensor informs the ECM that the post-TWC air-fuel ratio is lean (low voltage, i.e. less than 0.45 V).
Conversely, when the air-fuel ratio is richer than the stoichiometric air-fuel level, the oxygen concentration in the exhaust gas is low. The heated oxygen sensor informs the ECM that the post-TWC air-fuel ratio is rich (high voltage, i.e. more than 0.45 V). The heated oxygen sensor has the property of changing its output voltage drastically when the air-fuel ratio is close to the stoichiometric level.
The ECM uses the supplementary information from the heated oxygen sensor to determine whether the air-fuel ratio after the TWC is rich or lean and adjusts the fuel injection duration accordingly. Thus, if the heated oxygen sensor is working improperly due to internal malfunctions, the ECM is unable to compensate for deviations in the primary air-fuel ratio control.
The ECM continuously monitors the heated oxygen sensor output voltage while the engine is running. The sensor output voltage drops to below 0.2 V (extremely lean status) immediately when the vehicle decelerates, and the fuel cut is operating. If the voltage does not drop to below 0.2 V for 7 seconds or more, the ECM determines that the sensor response has deteriorated, illuminates the MIL, and stores a Diagnostic Trouble Code (DTC).

• Faulty Oxygen Sensor Bank 1 Sensor 2
• Oxygen Sensor Bank 1 Sensor 2 harness is open or shorted
• Oxygen Sensor Bank 1 Sensor 2 circuit poor electrical connection
• Gas leak from exhaust system