'98 3.8 oxygen sensor

I am getting service engine light and code 0135 "o2 sensor heat ckt malfunction". Bank 1 sensor 1.
Were is this sensor located? do you need special tools to work on it? can it be removed and replaced without having to take a lot of things on the engine apart or off the engine?
and maybe the most important question, how does this heater circuit work and how does the computer know that it is malfunctioning?
I reset the code, the light extinguished and stayed off for a couple of days. Then it returned with the same 0135 code.
I reset it again and it stayed off for over a week, then returned.
I reset it once more and had an annual safety inspection done, It didn't pass nor fail, it just said it wasn't ready yet. The attendant told me to drive it 80 miles then come back. I did and it passed, but a few days latter it came back on again.
It seems as if it works for a while and then fails, almost as if it has a bad connection or something of that sort.
can anyone give me more info on this trouble? Or past experience and knowledge?
Thanks Rex.

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.

Thanks Airjer for the info.
It sound as if the sensor needs to be replaced to correct the trouble.
I'll have my work cut out for me this weekend. I replaced the upper intake manifold gasket a few years back. It gave me a good idea of just how tight the area behind the engine is to work on. Do you have any suggestions on how to proceed to remove the sensor?
Thanks Rex