I may have answered my own question... any other opinions?
Originally called a "Lambda Sensor" when it was first used in fuel-injected European applications, the oxygen sensor monitors the level of oxygen (O2) in the exhaust so an onboard computer can regulate the air/fuel mixture to reduce emissions. The sensor is mounted in the exhaust manifold downpipe(s) before the catalytic converter or between the exhaust manifold(s) and the catalytic converter(s). It generates a voltage signal proportional to the amount of oxygen in the exhaust.
Could this mean my O2 sensors are bad? The car has 123,000mi on it and I am unsure if the sensors have EVER been replaced!