That one is the rear (next to the firewall) upper sensor (before the precat). It's also the hardest to replace. I was able to do mine a few years back fairly easily by renting a free O2 sensor replacement tool from AutoZone. The kit included an offset 3/8 drive socket that you just put a few ratchet extensions on, lay on your back and twist the sensor right off.
Before you do this you might do a search for "1131" and read about some of the other people here who have had the same code. Not all have been a defective O2 sensor.
