Electronic fuel injection (EFI) affords very precise control of fuel distribution to the individual cylinders of the internal combustion engine under both steady-state and transient operating conditions, thereby permitting exact control of exhaust chemistry. For a time, EFI fuel distribution was very accurately scheduled or preprogrammed on the basis of several sensed engine parameters. More recently, closed-loop control of electronic fuel injection, with its many attendant advantages, has been realized. For this closed-loop system, a sensor has been developed that measures oxygen concentrations in the exhaust gases and that outputs an electric voltage that changes sharply as the air/fuel ratio approaches the stoichiometric point. By sensing this unique operating point and feeding back signals to the electronic control circuitry, it becomes possible to so vary the fuel distribution that the A/F can be realistically maintained within a ratio range of ±0.1. The three-way exhaust gas catalyst used with the closed-loop control very efficiently reduces the concentrations of all three harmful emissions (hydrocarbons, carbon monoxide, and oxides of nitrogen) within this limited ratio range. The system has demonstrated in the laboratory (low mileage) 1976 emission levels and still provides good vehicle drivability, performance, and fuel economy.