The oxygen sensor on a vehicle is a vital part of the emissions control system that is responsible for minimizing the amount of harmful exhaust gases. It is responsible for the monitoring of such things as catalytic converter operation, and it gives the Electronic Control Module (ECM) vital data regarding the operation of the engine -- data that is used to make adjustments that can result in cleaner burning of the fuel.
Every car that has On-board Diagnostics (OBD) has at least two oxygen sensors, and sometimes up to five. These sensors, which are located in the stream of the exhaust system, deliver a voltage which varies according to the oxygen content of the exhaust. In this way, the ECM computer (or "brain" of the car's self monitoring system) can monitor conditions in the combustion chambers of the engine.
When the oxygen sensors detect an increase in the concentration of oxygen in the exhaust (indicating lean burning conditions), the voltage returned to the ECM decreases (to about 0.4 volts). When the engine is running rich and the oxygen content is low, voltage is increased (to somewhere around 0.7 volts). The ECM can then adjust the amount of fuel that is injected and thus regulate the engine's operation. If the ECM is unable to achieve optimum burning of the fuel, and the oxygen sensor consistently returns a voltage outside the normal operating range or the sensor is faulty, the "Check Engine" light will turn on, alerting the driver that there is a problem.
The primary oxygen sensor (also called the front O2 sensor) is located in front of the catalytic converter. The secondary oxygen sensor (or rear O2 sensor) is located further downstream in the exhaust, and can be used to monitor the operation of the catalytic converter. These sensors are called catalyst monitors. Some oxygen sensors are heated, and there are two types, the zirconia oxygen sensor and the titania oxygen sensor.
The operation of an oxygen sensor can be inhibited by contamination by silicone deposits (from antifreeze, RTV sealers or waterproofing sprays) or carbon deposits. If this happens, it can upset the fuel injection system. Replacing the oxygen sensor at regular intervals (after 100,000 miles for a heated three wire sensor, and after 60,000 miles for a one or two wire sensor) can prevent this from happening.
Since their debut in the 1970s, emission control systems have been responsible for preventing millions of tons of contaminants from entering the atmosphere by enabling cars to burn fuel more cleanly and get better gas mileage. One of the chief players is the oxygen sensor. Without it, the car's computer has no way to regulate the burning of the fuel, which would result in more harmful emissions. In addition, without the oxygen sensor, faulty catalytic converters would go undetected (at least until the car's next inspection).
