[Airjer_]

The upstream O2 sensor is in the rear exhaust manifold. Look down in between the rear valve cover and the fire wall and you'll see it. The way it knows it is "bad" is basically the PCM looks for the amount of time the O2 takes to become active. If it doesn't start switching than it knows the heater is not working. The heater is powered up through the ECM1 fuse which controls other items along with the heater for the rear O2 so its safe to say that the fuse is probably ok.