The O2 sensor sends an electric signal directly in proportion to the oxygen content in your vehicle's exhaust stream. The higher the oxygen content in the exhaust stream, the lower the voltage signal the O2 will produce, and visa-versa. Through this information the emissions computer can determine whether to deliver more or less fuel to the combustion chambers.
Late model vehicles are equipped with two oxygen sensors, one before and one after the CAT in order to monitor oxygen content exiting the exhaust manifold and at the same time to ensure the Catalytic Converter/s are functioning properly.
Heated O2 Sensor: The more recent model of the oxygen sensor is the heated oxygen sensor (H2O Sensor). An oxygen sensor's temperature must be around 650F before it will generate a voltage signal. By adding an internal heater to the oxygen sensor, voltage can be generated long before the engine has completely warmed up. Most oxygen sensors in newer cars are of this type.
Location: Oxygen sensors are always located along the exhaust path and before the tailpipe. In vehicles with only a pre-CAT O2 sensor the sensor will be located either directly before the catalytic converter or on the exhaust manifold. In vehicles with both a pre CAT and post CAT oxygen sensor, one sensor will be located directly before the CAT and the other directly after. Vehicles with dual exhaust systems can have up to four oxygen sensors monitoring exhaust streams on both sides of the engine. Dual bank and dual exhaust engines are typically V-6 or V-8 models.
For the Smog Test: Fuel delivery to an engine's combustion chambers are controlled by the vehicle's emissions computer or engine control unit (ECU) and fuel injectors. In order for an engine to pass a smog inspection, fuel delivery must be preciously controlled to produce the least emissions. If the injectors do not present adequate fuel to the combustion chambers, this causes low CO emissions and a lean fuel mixture causing high HC. In a situation where the injectors present too much fuel, this causes high CO emissions and a rich fuel mixture resulting in high HC emissions.
The main component responsible for letting the computer know how much fuel is in the exhaust system at any given moment, is the Oxygen Sensor. The Oxygen Sensor sends an electrical signal to the ECU, letting the computer determine exactly how much fuel it should continue to deliver to the combustion chambers. If for any reason the 02 sensor does not send accurate information to the computer or is lazy in sending the correct signal, the fuel delivery program will be altered.
Operation: During normal operation an oxygen sensors voltage should be switching between 0.1 to 1.0 volt at a rate of approximately 50 cycles per minute. An O2 sensor voltage above 0.45 volts is interpreted by the ECM as a rich exhaust, and an O2 sensor voltage signal below 0.45 volts as a lean exhaust.
The ECM's responsibility is to keep the oxygen sensor voltage switching between high and low voltage for optimum fuel efficiency, lowest emissions and highest miles per gallon. Usually the first sign of a damaged or defective oxygen sensor is poor fuel economy.