What does the oxygen sensor signal represent?
	The amount of voltage the sensor generates is proportional to the difference in the percentage of oxygen between the outside air and the exhaust gas. A rich air fuel mixture will have less oxygen in it, so a high voltage is generated. A lean air fuel mixture will have a higher percentage of oxygen, so the difference between it and the outside air is less and a lower voltage will be generated. The sensor’s voltage signals will vary between 100 and 900 millivolts. The engine control module (ECM) applies a 450-millivolt reference voltage to the sensor. The ECM compares the reference voltage to the voltage being generated by the sensor to determine if a rich or lean air fuel mixture is needed. A voltage below 450 millivolts indicates a lean exhaust, so the ECM sends a rich command signal to the fuel delivery system. A voltage above 450 millivolts indicates a rich exhaust, so a lean command is sent to the fuel delivery system.
	What is the oxygen sensors responce time to changing A/F ratios?
					O2 Sensor Cross Counts: During normal engine operation, the O2 sensor voltage is constantly fluctuating between 100 and 900 millivolts. This fluctuation reflects the changes in the air fuel mixture as the ECM controls the fuel delivery system. How often the O2 sensor voltage crosses the reference voltage (450 millivolts) is referred to as cross counts. A good performing O2 sensor will cross the reference voltage from high to low and back to high in just a few milliseconds. During a forced rich / lean test a good sensor will drop from 0.9 to 0.1 volts in less than 100 milliseconds or a tenth of a second. When O2 sensors begin to degrade a slow reaction time from rich to lean shows up as fewer cross counts to the ECM. This condition causes higher emission levels and increases fuel consumption.