P0139 = secondary O2 sensor sluggish response
For your car (now that I know what year) that sensor is in the cat. Sensor is #31 in the picture. It's screwed into the side of the cat #2. The drawing shows a sensor #17 in the right position, but #17 is for a different year car. Your #31 sensor has a wire that goes thru the floor before you can unplug it. So you'll be rooting around under the carpeting, under the passenger front seat.
The front sensor is shown as #16, you don't want that one.