When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich.
Chemistry and physics are branches of science that both study matter. The difference between the two lies in their scope and approach. Chemists and physicists are trained differently, and they have different professional roles, even when working in a team.
Known as the central science, chemistry is the study of the properties of and interactions between matter and energy. Matter is stuff, and atoms are what make up that stuff. So you could say that chemists study all the stuff on Earth as well as what that stuff itself is made of!