"Sensor 2" is the rear-most sensor...the one after the cat. However, I'd suggest that you do a couple of things before you buy a new sensor. First, check the fuses and relays under the hood...if the fuse is blown, then the P0138 code will be thrown. If the fuse is good, then crawl under the Jeep and start checking the connector and wiring for signs of damage. Both of these checks will take you no longer than five minutes, and that's if you don't know where to look. If you cannot find any sort of simple cause for the heater circuit to be out, then go ahead and swap the O2 sensor. For that, I suggest Liquid Wrench, an O2 sensor socket and an impact gun...you'll avoid most of the hassle with that combination.
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