The catalytic converter reduces emissions most efficiently when the percentage of oxygen in the exhaust falls within a certain narrow range. The oxygen sensor system is part of a closed-loop system which regulates fuel mixture to maintain peak combustion efficiency and proper oxygen content in the exhaust. The oxygen sensor continuously measures the amount of oxygen in the exhaust stream and sends a proportional signal to the fuel injection system which in turn adjusts the fuel mixture. The oxygen sensor system consists of an oxygen sensor which is mounted in the exhaust system and linked electrically to a control unit and, in turn, to the fuel injection system.