I always like to have the factory service manuals. For my year (5 yrs. older than yours) there's a test for the oxygen sensors. On mine, there's an under hood "data link connector" with a flip up cover and labeled pins, including oxygen sensor. The test involves using a volt meter to read the number of voltage fluctuations over a short period of time and if the count exceeds a certain number the sensor is good. Your oxygen sensors are heated. Mine aren't. The heating circuit, is like a filament in an incandescent light bulb. They can burn out. My understanding is that this is one of the more common failure modes. The heating element helps the sensor reach operating temperature more quickly. Apart from the heating element, my understanding is that oxygen sensors degrade slowly over time primarily from carbon accumulation. My theory, is that if the car is kept in good condition, (don't know if your model has spark plug wires or "coil over" set up or even a distributor) on mine, I've replaced the distributor cap, rotor, and spark plugs, removed and completely adjusted and cleaned the throttle body including the vacuum ports to the EGR modulator. I also use Red Line complete fuel system cleaner in every tankful which promotes cleanliness including emission control components. When I replaced the oil pan gasket and removed the section of exhaust pipe for access, I could see the second O2 sensor and it was completely free of any carbon. Also the main oxygen sensor still tests significantly more voltage fluctuations than required in the manual, so although I was considering replacing it just based on mileage alone (apx. 140,000), I have not. I once replaced an oxygen sensor on a Mercedes, just to "help it run better" as a "maintenance item" and could detect zero difference. The newer O2 sensors are more sensitive and efficient. As you may know the oxygen sensor monitors the exhaust stream to feed data to the ECM (electronic control module aka: engine computer) to continuously adjust the fuel mixture at the injectors. One of the reasons I like the Red Line product, is that their technical data section explains that if one injector is slightly off, the O2 sensor at the exhaust manifold is altered richening the mixture at all injectors to compensate and raise the average reading at the exhaust manifold, lowering efficiency and fuel economy.