Jim....I can't think of any new car that doesn't have 4 sensors. It's an EPA requirement. The rear two are after cat and do nothing more than monitor the efficiency of the cats. They will throw a code when they go bad.
Next, like Jim said, O2 sensors do wear out. They are a sensor that generates it's own signal. It's a very small voltage (less than 1 volt). There is no power to the O2 sensor other than the heater that is used to get the car into closed loop quicker. (O2 sensors need to be hot to operate) As the sensor reads the amount of oxygen in the exhaust, it sends this small voltage signal to the ECM. As the car switches from lean to rich and the ECM makes corrections, the O2 sensor reads from above 4.5 volts to below 4.5 volts (appx) at a very high rate of speed. This is called cross counts. When the O2 sensor gets tired, it can no longer make the switches as fast as it needs to and you get a code showing the sensor is slow and needs to be replaced. If the sensor fails completely, you get another code indicating that. Complete failure is rare. If your sensor is reading lean, that by no means indicates a bad sensor. The sensor may be (and probably is) reading correctly and you have, as Quik suggested, a vacuum leak. Only a proper diagnosis with the equipment that can monitor the sensor will tell you for sure. With that equipment, you can measure cross counts and force the engine rich or lean and watch the sensor's reaction. If your car has over 50K on the sensor, I would replace them as routine maintenance...at least the front two. But don't be surprised if you get your CEL back.
