All post-1996 vehicles have a lot of emissions equipment, including oxygen sensors. Most emission-controlled cars have two to four oxygen sensors, depending on the amount of cylinders and exhaust pattern. These sensors are broken down into two categories, upstream and downstream.
The main function of the oxygen sensor is to act like the nose of the vehicle. They take readings from the exhaust and convert them into data for the vehicle's computer to process. The computer then decides how to adjust fuel, air and other components to keep pollutants to a minimum and engine performance at a maximum.
The upstream oxygen sensor is located before the catalytic converter. This measures the level of pollutants coming directly from the engine. It also senses any raw, unburned fuel coming from the combustion chambers.
The downstream oxygen sensor is located directly after the catalytic converter. This sensor measures the pollutants making it through the converter and out of the tailpipe. The data from this sensor is compared to that of the upstream sensor. If the data between the two sensors becomes more similar, the vehicle's computer may trigger a catalyst inefficiency check engine code, which turns on the Check Engine light.