The normal aging process will eventually cause the oxygen sensor to fail. However, the sensor, may also fail prematurely if it becomes contaminated with lead from leaded gasoline, phosphorous from excessive oil consumption, or silicone from internal coolant leaks or using silicone sprays or gasket sealers on the engine. Environmental factors such as road splash, salt, oil, and dirt can also cause a sensor to fail, as can mechanical stress or mishandling.
A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode f operation, resulting in much higher fuel consumption and emissions.
An additional consequence of an O2 sensor failure may be damage to the catalytic converter. A rich operating condition causes the converter to run hotter than normal. If the converter gets hot enough, the catalyst substrate inside may actually melt forming a partial or complete blockage. The result can be a drastic drop in highway performance or stalling because of a buildup of backpressure in the exhaust system.
Home or professional auto repairs that have used silicone gasket sealer that is not specifically labeled "Oxygen sensor safe", "Sensor safe", or something similar, if used in an area that is connected to the crankcase. This includes valve covers, oil pan, or nearly any other gasket or seal that controls engine oil. Leaded fuel will ruin the O2 sensor in a short time. If a car is running rich over a long period, the sensor may become plugged up or even destroyed. Just shorting out the sensor output wire will not usually hurt the sensor. This simply grounds the output voltage to zero. Once the wiring is repaired, the circuit operates normally. Undercoating, antifreeze or oil on the *outside* surface of the sensor can kill it. See how does an Oxygen sensor work.
This info was stolen!!! from this site.... when my o2 sensor was broke, the check engine light indicated bank 2 was running lean, so the computer pumped 30% more gas trying to fix the signal that it was getting from the bad sensor. So in actuality, the engine was running extremely rich cuasing egg smell under hard acceleration and pathetic gas mileage....hope this helps some at least......
)), let me research a bit more and i'll post again later...