Car engines produce power by burning gasoline in the presence of oxygen from the air around us, and the O2 sensor, or oxygen sensor, is a critical component in the proper operation of almost all cars manufactured since about 1980. This sensor, mounted in the exhaust system near the engine, detects the level of oxygen in the exhaust gasses and sends a signal to the engine management computer. The computer can then direct the fuel injection system (or carburetor, on older cars) to change the amount of fuel introduced into your intake system. This alters the air/fuel ratio, which directly affects the efficiency of your engine.
The air/fuel ratio is simply a measurement of the amount of air compared to the amount of fuel, expressed as numbers. With gasoline as the fuel, the ideal air/fuel ratio is 14.7:1, meaning 14.7 times more air than gasoline. When burned in this ratio, optimal power is produced under most conditions. Different engines and different circumstances may require slightly different ratios, but 14.7:1 is considered ideal under most conditions. Anything above 14.7:1 means excess air is being introduced into the system, which is referred to as a lean mixture. Anything below 14.7:1 means excess fuel is being introduced into the system, which is referred to as a rich mixture.
If your car is running lean, then all the fuel in the combustion chamber is consumed but there is excess oxygen remaining. When the O2 sensor detects excess oxygen in the exhaust gasses, it sends a higher-than-normal voltage signal to the engine computer. A lean mixture causes the engine to run hotter, reducing efficiency, and excessively lean mixtures can actually damage spark plugs and melt pistons. This can quickly result in costly engine repairs.
If your car is running rich, then all of the oxygen in the combustion chamber is consumed. In this case, the O2 sensor sends a lower-than-normal voltage signal. A rich mixture also reduces efficiency and can coat the spark plug and the inside of your combustion chamber with carbon deposits, further reducing performance.
When operating properly, the O2 sensor cannot cause your engine to smoke. If the sensor is defective, however, it can cause your car to run either rich or lean. If your car is running excessively rich, you may notice black smoke coming from your tailpipe. If your car runs excessively lean for an extended period of time, you could cause serious engine damage, resulting in black, white or blue smoke from the exhaust, but usually you will be alerted to other symptoms first, such as rough running. Fortunately, the computer systems in modern cars will usually alert you with a warning light on the dashboard when the O2 sensor is not operating within normal limits.
