About O2 sensors:

An O2 sensor (lambda sensor) has an expected life span of about 30,000 to 50,000 miles. Sensors may fail prematurely if clogged with carbon, or contaminated by lead from leaded gas or silicone from an antifreeze leak or from silicone sealer.

O2 sensors become sluggish as they age. Eventually the signal does not change at all. When this happens, the EML and/or check engine light may come on, and the engine may experience drivability problems or damage caused by an overly rich fuel condition. High fuel consumption, high CO and HC emissions, poor idle, and/or hesitation during acceleration are typical complaints.

If the average voltage from the O2 sensor is running high (more than 0.50V), it indicates a rich condition, possibly due to a bad MAP, sensor or leaky injector. If the average voltage reading is running low (less than 0.45V), the mixture is running lean possibly due to a vacuum leak or because the sensor itself is bad. If the O2sensor continually reads high (rich), it will cause the engine computer to lean out the fuel mixture in an attempt to compensate for the rich reading. This can cause lean misfire, hesitation, stumbling, poor idle, high hydrocarbon emissions (from misfiring) and engine damage.

If the O2 sensor continually reads low (lean), it will cause the engine computer to enrichen the fuel mixture. Injector pulse width will be increased causing fuel consumption and carbon monoxide emissions to go up. Especially in a performance tuned vehicle, a constantly rich fuel mixture can cause the catalytic converter to overheat and it may be melted.