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Re: MIL light on
P0139 Secondary Heated Oxygen Sensor (Secondary HO2S) (Sensor 2) Slow Response This is the Catalytic Converter efficiency monitor, it is reporting that the Cat Conv is not working as it should (cleaning up the exhaust), how many miles on the car?
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The code refers to an oxygen sensor that is no longer detecting changes in fuel delivery by way of sensing exhaust oxygen content. Bank 1, sensor 1, tells you which sensor is bad, in this case, it's the sensor that is mounted before the catalytic converter, on the rear bank of cylinders as you look into the engine compartment. It would really be a good idea to change both pre-cat O2 sensors at this point as they are both twelve years old.