An O2 sensor, also known as an oxygen sensor, is an electronic device that measures the proportion of oxygen in the gas or liquid being analyzed. It is a key component of the exhaust system in modern cars and helps regulate the engines air-fuel ratio. The sensor checks the amount of oxygen in the exhaust compared to the amount in the air and feeds that information to the engine computer, which then adjusts fuel and air intake. By monitoring oxygen levels, the sensor provides a means of measuring fuel mixture, and it lets the computer know if the fuel mix is burning rich (not enough oxygen) or lean (too much oxygen). O2 sensors became mandatory on vehicles starting in 1981, and many vehicles made after 1996 were required to have multiple oxygen sensors. The additional O2 sensors are used to monitor how the catalytic converter is operating. The O2 sensor can be found within the exhaust manifold, which is connected to the engine and helps collect emissions. A failing oxygen sensor could lead to the engine control unit being unable to adjust the air-fuel ratio correctly, resulting in lower gas mileage, sootier exhaust, hesitation when stepping on the gas, stalling, and rough idling.