An oxygen sensor is a device used in vehicles to measure the amount of oxygen in the exhaust gases. It helps the engine control unit (ECU) adjust the air-fuel mixture for optimal combustion, improving fuel efficiency and reducing emissions. Typically located in the exhaust system, there are usually two types of oxygen sensors: upstream and downstream. The upstream sensor monitors the air-fuel mixture before it enters the catalytic converter, while the downstream sensor checks the efficiency of the catalytic converter itself. Proper functioning of these sensors is crucial for maintaining vehicle performance and meeting environmental standards.