The oxygen (O2) sensor is the master switch in the fuel control feedback loop. The sensor monitors the amount of unburned oxygen in the exhaust and produces a voltage signal that varies from about 0.1 volts (lean) to 0.9 volts (rich). The computer uses the O2 sensor's signal to constantly fine tune and flip-flop the fuel mixture so the catalytic converter can do its job and clean the exhaust.

If the O2 sensor circuit opens, shorts or goes out of range, it usually sets a fault code and illuminates the Check Engine or Malfunction Indicator Lamp. However, some O2 sensors that are badly degraded will continue to function well enough not to set a fault code, but not well enough to prevent an increase in emissions and fuel consumption. The absence of a code or warning lamp, therefore, doesn't mean the sensor is doing its job.

The performance of the O2 sensor tends to diminish with age as contaminants accumulate on the sensor tip and gradually reduce its ability to produce voltage. This kind of deterioration can be caused by a variety of substances that find their way into the exhaust such as lead, silicone, sulfur, oil ash and even some fuel additives. The sensor can also be damaged by environmental factors such as water, splash from road salt, oil and dirt.

As the sensor ages and becomes sluggish, it may not allow the computer to flip-flop the fuel mixture fast enough to keep emissions within acceptable limits. If t...