Let's start over on the explanation of your oxygen sensors.
You have four sensors. Only two of them are used for determining fuel mixture. Those would be the upstream sensors. The downstream sensors are used to determing the health of the cats.
Bank two happens to be the front bank, the bank closest to the radiator.
So you are looking for the first oxygen sensor on the front bank. It would be just in front of the cat that is built into the exhaust manifold.
MIL eliminators are only used on the downstream sensors. The downstream sensors are mounted just behind the cats in the exhaust manifold. On our cars, the downstream sensors do not monitor the main cat which is a little further back after the two pipes join into one.
MIL eliminators are needed if the cats in the exhaust manifold are disabled (gutted) or removed (headers installed instead the the stock manifolds).
The upstream sensors give an instant voltage signal to the computor telling how rich or lean the system is. The sensor generates voltage in the absense of oxygen. The range is .1 volts (lean) to .9 volts (rich). The voltage is constantly switching throughout the range because the exhaust pulse contains different mixtures depending on the part of the combustion cycle the cylinder is in.
The downstream sensor with a healthy cat will move much more slowly since the cat evens out the mixture (or buffers the mixture). If the downstream sensor switches at about the same rate as the upstream sensor, the computor knows that the cat is not healthy (or missing). The portion of the computer that checks the downstream sensor reading (cat monitor) does not run all the time. It only checks the cats after certain driving conditions have been met.
With all of that said, most important of all, is that an oxygen sensor code may not mean that the oxygen sensor is bad (in fact, most of the time oxygen sensor codes are from something other than bad oxygen sensors). The codes are not the final answer, they are only the gateway for testing and indicate the general area that needs to be tested. You may have a vacuum leak or dirty injectors for example.
