[dcb]

From an EPA perspective, the rear sensor is there to double check that the front sensor is being responsive, and so it needs to be heated so it can tell when the front sensor comes "on line". It is also to see that there is actually less O2 downstream of the cat, to make sure that the cat is actually doing something. But the sensor immediately downstream of the engine should be more accurate and thus the primary sensor.

FYI, it may never run the obdII o2 sensor/cat converter readiness monitors if you have a heated front sensor and always heat it up before starting the engine.